You aren’t being dismissive by suggesting she discuss this with your MIL. And your SIL knows that is what she should do. She is drawing you into this situation…and really, I know you want to be supportive of SIL…but this is SILs situation…not yours.
At this point, MIL wants that table. So really at this point, that ends the discussion. Talking about Bob dying first probably isn’t going to score any points with MIL. Talking about the “what IF” Bob dies first won’t score any points.
I think you are doing the best you can to be supportive, but sometimes you may need to step away.