I've been taking care of a gentleman since Feb 2009. When his sisters call they never ask me questions. Should I ask them if they have any?

If this gentleman hired you unless he has given you permission to discuss his care with his sisters or anyone else you can not discuss anything with anyone.
If they have questions about his health or anything “private” they need to ask him.
You should be keeping a log or journal as to day to day events. This can be shared if necessary with whoever is POA.

I do feel I have to comment on your profile.
He should be paying you. Living in a room in his house is NOT pay for the care you are giving.
It does not matter that you may inherit property. There is a very good chance that any Will will probably be contested.
I truly think you need to discuss this situation with an attorney

I would like to think that if they had any questions for you, they would have asked them long ago. I don't think it takes a rocket scientist to figure out that if they haven't asked any questions in 12 years, they probably never will.
Just keep doing a good job with this gentleman, and let him know that at least you care about him, as it's obvious his sisters do not.

Who hired you and who do you report to about his health and needs? If it's not these sisters then they don't need to know anything from you, their phone calls are to visit with their brother not to assess his health and your abilities. You should be keeping a log of your day to day activities and that should always be available to the person in charge of his care, no one else.