Care plan meeting at rehab - nonfamily participant. Am I wrong to be ticked off?

Your dad's fine with the friend attending.
The facilty is fine with the friend attending.
You do say you've been able to ask a lot of questions, so that need is being met. You're sound unhappy that your sister isn't doing the meeting in a way you prefer. However, you're not the one in town managing the day-to-day.
I bet it's crystal clear to your sister that you don't think she's the ''sharpest or the most assertive knife in the drawer'', and has actually taken steps to address this by having a friend with experience support her. I say God bless that good friend, who's willing to take time from their day to be of help her. You should be grateful for every bit of support she can get. Support of her helps her support dad.
This seems more about control and trying to maintain a family pecking order. Ask yourself if it's worth undermining your sister's decisions and adding extra stress to what should be a routine meeting. Extra input can always have some value, and if it helps your sister have a better handle on things where's the harm?

Yes, I think you are wrong not to want her there, but I think you are ALSO right in the way to handle the situation.
In fact, WHEN POSSIBLE I always advise people to bring a friend or a tape recorder to meetings to discuss all these things, and especially to discuss diagnosis and treatment. We get anxious. We don't hear. We miss the important questions to ask. I would, were I you, be gently honest about your feelings (they are proper and reasonable) at the meeting, that the friend is there for support and to help your sister, but that the family's questions should be allowed first if there is a time limit, and then the friend might suggest, having "been here" things that your Sister and you may need to know. This should be on the table. Do know that not all facilities even allow "outsiders" and your Sister may have to sit with friend BEFORE the conference, to tell her everything she can think of. And do know, no matter how many friends show up,often things are missed in any case. Good luck.

1. Rediculous. Friend does not belong in the family meeting.

2. Your Father is YOUR Father, not hers.

3. Her/his relevant experience is just that: HER experience. May not be relevant to your situation anyway.

4. The PT, Doctor, other Professionals present have professional experience PLUS the facts on your Father's medical history & progress.

5. If your Sister wishes to have moral support or advice from her friend, she can, later, with friend at a cafe etc.

I could keep on..

Unless the best friend is also the best friend of the person needing care, or unless Sis is engaged to best friend, yes I would be angry and PO'ed too. If she's just a friend, then what's keeping YOU from bringing your bestie? How about that bestie's other friend?

Personally I would not allow her to be in the meeting.
There will things that may be discussed that are private, as a matter of fact the entire conversation is about PERSONAL health and physical concerns. To allow someone that is not included on the HIPAA forms is a violation and since this person has no personal involvement they have no right to the information.
And you have no way of knowing if she will keep this entire meeting confidential, she is not bound by any regulations.
You can be ticked off. I would be ticked off. If the entire family agrees that this person can be involved you can request that this person hold any and all comments and questions to the end of the meeting.
I would also request that they leave the meeting if at anytime they begin to argue with a member of the family or the staff.
If they disagree with any medical decisions the family makes I would ask them to leave.

My care meetings were exactly 15 min of the staff telling me what was being done for Mom. How she was doing in therapy, her meds, etc. When I tried to ask a question, I was told they had someone waiting for their meeting. I told them since it was my first meeting, I should have had longer to ask my questions.

I don't think the friend should be asking any questions. And if more than one family member is going to be there, one should be made the spokesman. Maybe getting together to make a list of questions.

I got most of my answers from the nurses and the DON when I visited.

I’m going to play devil’s advocate here.

Is it possible the concerned outsider might have emotional distance and keep her emotions in check during an emotional meeting? Could this result in better results?

Is it possible the concerned outsider might have been in a similar situation before that could cause her to be an effective advocate for the patient and family?

Is it possible the family members are simply hostile to each other and would end up fighting or competing with each other instead of focusing on the patient and his care?

The whole point of these meetings is to get the best care for the patient as possible. The family are only a part of it because their involvement will affect the patients outcome for good or bad.

Infighting between family can be a harbinger of bad outcomes for the patient.

Honestly, sometimes it is truly a plus to have someone less emotionally vested to be there as well. I know My emotions get the best of me when I am trying to get answers to those hard questions. If I didn't have my wife with me during some of those meetings, I am not sure how I would have reacted. Yes she's family but she is also looking at it from a more clinical view than I can, because It's my dad. I love him, and the hard questions KILL me because I see the man that raised me. Not the person that is in front of me now who needs strong support and tough love to remain safe and healthy :(.

I may be off base here, but it seems that your sister wants her friend there because SHE needs support. Has your sister ever indicated she feels “bullied” (maybe too strong a word) or “not heard” in the family or “outnumbered”?
I’m not saying that’s true, but it could be her reasoning she feels the need to have someone in “her corner”.

Personally, I don’t feel a non family member needs to be there. However if indeed she has been thru this before, she may have something helpful to contribute.

If possible, delicately (if she oversensitive) ask your sister why she feels the need to have her friend in the meeting? Can her friend write down some things and give the list to your sister to inquire about during the meeting? That way, it’s kept to the family, but your sister gets “help” from her friend.

Ask the nurses, however I bet they say whoever the family wants can attend.

Can you (again delicately) ask your sister why she wants non family involved?
Doesn’t she believe the rest of you can ask pertinent questions?

This is such a hard time for any family. It doesn’t need to be an added layer of difficulty dealing with someone who is not capable of handling things without a friend present. After all, family should be there to support each other.

Yes. You are wrong to be ticked off. Why would you object to your sister's friend going to the care plan meeting? She's there to help and could be a good a resource to all of you because she went through what you all are going through now. Her experience will be helpful. I wish we had a friend like this when it was time for my father's care plan meeting with the nursing home. My sister and I were flying blind because we really didn't understand all the dishonesty and thieving treachery a nursing homes engage in. I got educated very quickly though.
I offer to help anyone I know who has to deal with a nursing home because I know firsthand what they will pull on a family. So you and your family are lucky that your sister's friend is willing to be another pair of ears at the care plan meeting. You can always use one.