My wife and I (both 36) have been together for 15 years, married for 8. It hasn't always been the easiest time, esp. when it comes to my in-laws. She can be very defensive of them, no matter how many times, and in how many ways, they hurt her. This specific case involves her baby sister, who annoys us the least.
Recently, SIL and her fam have run into some hard times. No details here, but my wife volunteered us to babysit for them twice per week, 5-6 hours per day. These kids (5 & 3) are difficult, and our kids (4 & 2) don't really get along with them. But because it's a family emergency, I agreed.
The conflicts started with how long this would go on for – I thought maybe up to a year (or whenever SIL & hubby could make other arrangements), my wife says she's OK doing this forever ('It's for my family. What are we doing on our weekends anyway?'). She said if I'm not willing to continue, she'll keep it going alone; that's a disaster waiting to happen. That discussion didn't go very far, and on my end, that issue was never resolved. So I sated myself to mentally reframe it as a 'wait and see' situation.
In the time since, we're still babysitting, and my wife cooks/buys them extra meals/groceries (not just the kids, SIL too; basically shopping esp for them) at least once per week. A little annoying, but I suppose arguably an extension of our babysitting. But then she also complains about money shortages… Oh well, it's for her baby sister, right? I'm also worried about their kids harming our kids (accident or otherwise), property, and/or our rented home, but my wife has been dismissive of my concerns ('Why worry about it if it hasn't happened yet?', 'We'll cross that bridge when we get there', 'We'll just clean/fix it, NBD'). Not great.
My wife has since expanded our 'offerings' to include using our laundry machines whenever they want (NBD, I guess), emergency anytime drop-in (in case they lose their keys or something), among other things. Annoying, but understandable.
Here's where I'm really nettled. We've been saving money for a down payment on a home, and we've already disagreed on cost vs features (she's particular about the kitchen, yards, # of living rooms, etc.; I only ask 2 bathrooms and a price we can afford), which is hard enough. But now she and SIL have been talking about looking for a split home or a home with a granny cottage (Google 'ADU'), for all of us to share. I'm not down with this for a number of reasons (money, raises wife's bar on features, these situations always cause bad blood, it'll crowd us and our kids, local codes, it opens the door to other in-laws asking for 'favors'), but I'm not sure of how to tell my wife no without her getting mad and smearing me as a heartless jerk. Can anyone offer any tips? Thanks.
TLDR: My wife just can't seem to say no to helping her baby sister in need, and it's escalating to offering to share our future home with them indefinitely. How would I tell her no without BTA?