Thanks for the multitude of advice on my different scenarios.
The River House: Worth $1.2M; built in the 30's; owned equally by both sons now and both sons are responsible for utilities, upkeep, taxes etc. My DH is off 4 days a month so we rarely go down as I find it boring lol. I prefer the beach and the river house is in a lovely remote area with no wifi or cable... Now when he gets his boat after I get the countertops

then maybe. My MIL talked to both me and DH before she died and said this "takeover" was going to happen and DH needed to speak up. She asked me to make sure his voice was heard (because I am loud?). When I went this past weekend, I made sure that things were cleaned up as I still think of it as my MIL's home and want to give it the respect that it deserves.
The property has a vaulted garage that has an apartment with 3 bedrooms and a bath for guests. Apparently, my DH and his brother decided to replace the steep staircase and build one that is monstrous with landings. They did not pour footers and were using above ground footers? And a building inspector happened to drive by and shut the project down. He said they needed a building permit and to submit plans. OMG. So the access to the garage apartment is limited to a metal ladder type staircase for the time being. OMG I took one look at that unfinished thing and just about died. My MIL would be having a fit. The real reason for building the monster staircase is so my SIL can have her golf cart parked there... SMH.
Unlike my SIL, I have no plans to interject myself unless it affects my DH. She wanted to rip out all the flowering shrubs and flowers that my MIL had planted along the driveway that are beautiful because they are too much work. I told her she needed to ask DH about that first and that kinda shocked her back into reality. The house is not hers...