What if your spouse and sibling don’t get along and their interaction is inevitable?

thriftstore perhaps your presence is a good thing too keep peace harmony and sanity in the household

Talk to both. Set expectations to be “adult” (at a minimum). Start punishing one or the other if it can’t happen.

Tell them to settle their differences without you. It is their problem.

Tell them how it effects you, possibly?

Why don't they get along?

Don't go to family gatherings together

I only have my mother and sister. I’m not willing to give them up for anybody so they get it together or suffer.

Id tell them to shake hands