At 92 my mother-in-law is ready for AL. She is very clear thinking, but a fall risk. One family member wants to move her 4 hours away!

You say that Mom is "very clear thinking".
Then the decision belongs solely to Mom.
I am certain that you have mentioned to her that you will be able to get there not a whole lot, and that she will perhaps miss seeing the grandkids and great grandkids?
There has to be something we are missing here, that Mom is willing to leave the area she has lived in for some time with you all there and the kids and grandkids, and etc. I am wondering why, if she is clear thinking, she thinks this is a good move? Is she being offered to move in with your other sibling?
Do you siblings all get along? Have all of you discussed this with the sister who would like Mom closer to her? Why does she think this is a great move for Mom?

why wouldn’t she have a chance to see her grandchildren and great grandchildren? She’s going to assisted living not prison. She will allowed to leave the AL and have visitors there too.

its really her choice to make. It’s great that you are a caring daughter-in-law but the decision is really hers to make.

Ditto to the 2 prior comments "she can make her own decision" but she will need to grasp the reality of reduced visits. Four hours one way is very far away, especially when you have school-aged kids and all the weekend commitments that come with that. Does the sibling understand that they will become the primary caregiver at that point and all that it entails *in the future* as your mom's condition changes? Will this sibling be willing to provide accommodations for when people do visit on a weekend? 8 hours of driving in a single day is a lot for people with young kids. Staying overnight, buying meals when there's more than 2 people -- it all adds up. Everything sounds good in theory...and then reality happens. Hope your family can come to a harmonious agreement!