I think the best you can do is make tentative plans but allow for some variance. Before I even read your thought on reducing the 401k contributions, I had thought of that. Do great minds think alike? Or maybe we’re in the flock of crazy birds? (Birds of a feather flock together!)
Anyway, I don’t know how much that actual amount is that you’re contributing — but if you could temporarily reduce or stop it to buy out your brother that could seriously reduce or eliminate future debt.
I am not sure it matters in this situation but do you foresee you and dh getting back together? Will he be on the mortgage or deed of the family home?