What role, as a general rule, do the spouses of siblings have in decision making as it regards an aging parent?

It depends on you family dynamics, if the inlaws are close to your parents and have spent a lot time with them through the years it's totally different than if they are merely your sibling's spouses KWIM?
And if the discussions involve anything that would require a commitment of time or money from any of you then it's unfair to ask for commitments without including your sib's partners, because those decisions need to include everyone.

Here's the thing.

Some folks can be "insulted" about anything. Are theae folks both like that? If so, it makes for difficult family dynics no matter what the issue.

Do they think they have good information to share? Have they been down this road with their parents? Have some professional expertise?

Or are you calling on their help in doing the moving, explaining, money management?

One of my SIL's is a financial whiz and managed my mom's money. We involved her every step of the way

Since I receive no support from anyone in my family the input I receive from my spouse is invaluable. He is not bound by all the family of origin stuff that I am and
can deal with them like no one else. He can get them to do almost anything.
Shopping, banking, medical appointments, taxes etc. My parents are always on their best behavior when he is around. When they were looking into Assisted Living he took my Dad to all the tours because he knew I had reached my limit. All he asks is that we have a life together outside my parents. I suppose that sig others can be a problem but sometimes they have good insight into stuff that you don't see. They need a little input because their lives and possibly their money might be involved.

Carole, if you want family support in the future, it’s a good idea to include other family members in the discussions about what’s best. ‘Not your business’ isn’t a good way to start. ‘Tender-hearted’ ??

My suggestion would be to get close to a decision yourselves, then have a meeting to say ‘this is our current best option, do you have any other suggestions?’. Immediate family can still make the decision. Spouses live with the consequences of what’s decided- visiting, clearing out house, garden and selling, etc. I can see why they would prefer to be involved at some level.

If decisions impact them and their marriages, include them…

I am grateful I didn’t sit back and let decisions being made by my BIL, effect my marriage. His proposals would have had us doing the caregiving. All in the name of inheritance…

I typically agree that spouses are more peripheral to decisions. However, IF the MIL lives with one of the couples or IF one of the spouses does caregiving duties, then I feel differently. They deserve a place at the table to some extent. I would hope, though, that the son or daughter does most of the talking and giving input when dealing with other sibs.

I think it entirely depends on how the decisions made will impact their lives.

In your situation where you are placing your mother, I don't think spouses have must of a say.

But I would not discount that you and your siblings may be dealing with a degree of FOG, Fear, Obligation, Guilt, and they may have a clearer picture of the situation. Just from your post, you say Mum has ALZ and you are placing her in AL. It may be that memory care or a nursing home is more appropriate for her.

If there is discussion about the parents moving into one of their children's homes, then the spouse has as much say as anyone and more than the children who will not be providing care.

In my case, Dad lives in a suite in my brother and SIL's house. My SIL has far more say than I do. She has the burden of him in her home, I do not .

Spouses of siblings may not be the ones to make the decisions about care and placement, but they are certainly involved if the siblings are helping financially with Assisted Living and/or Health Care. Their input and support might be helpful, but maybe there are family dynamics here that we do not know about.

Since your siblings are married, their spouses ARE family. To exclude them will surely lead to problems down the road. Better to have open discussions but allow the final decision to rest with only a few people.