AITA for Refusing to Co-Sign My Sister’s Mortgage After My Parents Went Behind My Back?

I (28F) have been busting my butt for years to save for my own house. I work in software, so I make decent money, but it still takes forever to build up a good down payment. Meanwhile, my younger sister (25F) is in grad school with barely any credit. Our parents (both mid-50s) found a house near them and decided she needs it. They made an offer—without telling me—and now the deal only goes through if I co-sign.

The problem? I had no clue they’d do this. My parents basically dropped a bomb: “You have the best credit score—co-sign so your sister can get the house!” They also hinted I should chip in for the down payment because “you’ve got the money.”

If I co-sign, I’ll be on the hook if my sister can’t pay. She’s still in school, has debt, and zero backup plan. The bank might also reject my future mortgage application since they’ll see I’m already tied to another loan. But my parents say I’m “selfish” and “forgetting family values.” My sister’s calling me a monster for leaving her “stranded,” and my mom threatened to cut off any future financial help (like wedding money) if I don’t help right now.

Some relatives think it’s insane my parents tried to rope me into this after they already made the offer. Others say I should just do it for “the family’s sake.” I feel guilty, but also mad they put me in this spot. AITA for protecting my own finances, or are they wrong for strong-arming me into co-signing a mortgage I never wanted in the first place?

EDIT: I’m actually adopted lmao forgot to mention in my confused and angry state. My parents adopted me when I was very young because they’d been struggling to conceive. A few years later, they had my younger sister naturally, which was a huge deal to them—she was their “miracle baby.” Ever since, it’s felt like my role in the family became “the older, adopted one,” while she was the golden child who could do no wrong. Growing up, I was expected to pitch in more, be more responsible, and generally look out for my sister.

I worked my butt off in school, snagged scholarships, and eventually landed a good job in software. All the while, I felt like my family mostly saw me as the “fallback option” in case anyone needed financial or emotional support. Now that I’m actually building my own life—saving for a house, focusing on my career—I’m realizing how my success just makes me look like a bigger piggy bank to my parents. The more independent I become, the clearer it is that I need to separate myself from the constant guilt trips and the unspoken expectation that I’ll always bail them (or my sister) out. I love them, but I can’t keep sacrificing my own future to maintain a dynamic where I’m never the priority.

So thank you all for the wake up call. Update to come