Yikes. What a situation. First of all, even if your in-laws are "partial" owners, they need to remember that being a "partial" owner means having to work out suitable arrangements with the other owners. It does not give them the power to determine all the rules. Even landlords need to provide renters 24-hour notice before entering an apartment.
You not only made a considerable initial financial investment (did anyone else?) you also pay 80 percent of the mortgage. AND you occupy the premises.
You absolutely are within your rights to ask for advance notice for overnight guests--family or otherwise. As for non-family guests, these are a no-no without prior permission. Yes, it is their vacation property, too but just like a timeshare property, they need to have assigned weeks or weekends for visits so you can choose to be elsewhere. Should they be allowed to come and go as they please? No. Should they have unlimited access? No. Not when you're paying 80 percent of the mortgage.
They already have one bedroom to themselves that is off-limits to you.
But that is only the secondary problem. The real problem is that you are willing to leave your husband over this. While it is aggravating that your hubby is not dealing with this, you have to decide if your marriage is worth keeping. I certainly would not allow any pushy SIL to break up my marriage. She is your husband's brother's wife, not his sister, so it should be even easier to stand up to her.
Let them know that to make things fair, you all need to draft a suitable timeshare arrangement. SIL only has 10 percent rights. How many weekends a year should that allow her? When they are in the house, they can live by their rules. Otherwise, you don't have to follow her rules for where the dumb linens go and such. You live there. Be firm.
But be sure your own marriage is strong and intact by not making your husband feel bad for not standing up to them. You can do the dirty work if he won't. Don't blame him for the problems or take out your frustrations and anger on him. But be sure you are as sweet as honey to him so he'll remember whose side he's on and that it is in his best interest to keep you happy.
On the other hand, to keep the peace, I definitely would consider having the family buy you out of the house completely. Return your investment and cut yourself out of the vacation property rights. There can be only more pain in the future dealing with this SIL. I would hate to think what would happen should damages occur. (And by the way, who gets the tax right off?)
Find another home where you and your husband will live in peace in your own private domain.
