AITA for refusing to pay rent and saying I'm not responsible for my boyfriend's mom when the 3 of us are going to live together for a year?

So this is something new that came up recently. I met my boyfriend when we were 13 and started dating at 15. We've been together for 9 years now and I've always gotten along with his parents. They are very nice and I wish they were my parents. I wish I was exaggerating but that's how nice they've always been. They've always included me in family things and treated me like I was their own. I'm very spoiled; my parents are narcissists and don't care about me, and his parents like to make sure I know I has people that love me. So I've always loved them, but not to say they are perfect. They are very old-fashioned and want us to settle down. They are active Catholics but are not extremist about it. They want us to settle down and get married soon but not to the point where they've threatened to cut my boyfriend off if he doesn't; more just to the point that they wish he would. <br><br>Back to the point, I'm 22f and he's 22m. I moved out of my parents' house last year because I got a good job and my boyfriend and I moved in together. We live in a nice one-bedroom apartment and have a nice time. We make more money than we spend and we both are just trying to get good careers and figure out what we want to do with our lives.<br><br>About two months ago, my boyfriend's dad died. It was a heart attack and it was unexpected. His mom seems pretty sad but she's going to be okay. His dad left her pretty financially stable. <br><br>So his mom then talked to my boyfriend and I and said she had an idea. Right now she lives in a pretty big house not far from us. She gets lonely and feels too big in it but she doesn't want to move because it's where she lived with her husband. She suggested we all three live together for a year. My boyfriend and I would have our own separate bedroom. But it would give us a way to pay less rent since my husband's mom would be able to split the mortgage with other people. It would give my boyfriend more time to spend with his mom and help her with her grief. It would also give us an opportunity to live together and kind of like test it out for a year. We both agreed. <br><br>However, my boyfriend and I were a bit worried about money. We came up with a fair number by calculating rent + utilities and it was less than we were paying now for our one-bedroom. However, my boyfriend's mom said no. She said we needed to pay $800 a month for all three of us. We said it was too expensive but she said it was fair. I don't think it is. And my boyfriend's mom refused to pay for anything when she could clearly pay for everything. I'm still trying to talk to her, but my boyfriend is on her side.