My husband's parents had two houses. When they passed away they had a will leaving one house to my husband, and the other house to my husband's brother. Can they switch houses if both are agreeable? It needs to be all legal, etc. so they can put the houses in their wills. Thank you.

They can switch but after the will and probate are closed. Are both homes free and clear of all mortgages , liens and obligations? You may need to consult a tax attorney but I think they can sell each other the other house for one dollar but there will likely be other financial consequences like tax stamps and deed fees