AITA for refusing to co-sign a mortgage they secretly offered so my sister could buy a house?

My parents put in an offer on a house for my younger sister without telling me, then demanded I co-sign and hinted I should contribute to the down payment because I "have the money." I’m saving for my own home and don’t want to be financially liable for her mortgage. AITA?

I grew up adopted and always felt expected to be more responsible — my parents later had my sister naturally and she became the “golden child.” I worked hard through school, landed a well-paid job in software, and have been saving for my own down payment. My parents found a house near my sister, made an offer without telling me, and now the deal hinges on me co-signing. They framed it like it’s my duty because I have the best credit and "you’ve got the money."

I’m the sibling who saved, worked through scholarships, and built credit — and now my parents expect me to risk my future by co-signing a mortgage they arranged behind my back so my sister can move into a house I didn’t agree to help buy.

My parents told me they’d already made an offer and that the deal only goes through if I co-sign. They implied I should also chip in for the down payment, arguing I have the means. I was blindsided — I had no idea they’d do this behind my back. If I co-sign I’m legally responsible if my sister defaults, and lenders could count that liability against me when I apply for my own mortgage. My sister is still in grad school, has debt, and no clear backup plan.

"You have the best credit score—co-sign so your sister can get the house!"

When I pushed back, my parents accused me of being selfish and reminded me of "family values." My sister called me a monster for "leaving her stranded." Mom even threatened to withhold future financial support like wedding money if I don't help now. Some relatives told me my parents were unreasonable to spring this on me after the offer was made; others said I should just do it for the family. I feel trapped between protecting my future and being painted as uncaring.

"You’ve got the money — just co-sign and be a family person."

I love my family but I also fought hard to build my life, and I’m not willing to risk my ability to buy a home or be on the hook for a mortgage I didn't agree to. The adoption dynamic — being raised to be the responsible older child while my sister was the "miracle baby" — makes this feel even more unfair. Right now I’m standing my ground but feeling guilty and isolated.

🏠 The Aftermath

At present, the offer is conditioned on your co-signature and your parents are pressuring you. You haven’t agreed to co-sign (and haven’t committed to funding the down payment) — you’re protecting your credit and future mortgage eligibility. Your relationship with your parents and sister is tense: they view you as the one who should sacrifice, while you feel used and resentful.

Practically, if you refuse, the family may carry on without your help and the purchase could fall through, or your parents might try to find another way. If you co-sign, you assume real financial risk and potentially delay your own home purchase. Socially, refusing could deepen the perceived role expectations that have followed you since childhood.

Emotionally, you’re dealing with guilt from family conditioning, the sting of being treated like a piggy bank, and the desire to finally put your future first.

"I worked hard to build my credit — I can't gamble my future on a decision I didn't consent to."

You're trying to balance love for your family with reasonable boundaries that protect your long-term financial security.

💭 Emotional Reflection

This is a classic conflict between family obligation and personal boundaries. On one side, families sometimes expect those with means to support others; on the other, co-signing is a legal and financial commitment that rightly requires informed consent. Given your history as the adopted older sibling who carried extra responsibility, their demand feels like an entitlement pattern rather than a reasonable request.

You’re not being inherently selfish for prioritizing your financial future — buying a home is a major life milestone that depends on credit, debt-to-income ratios, and predictable liabilities. Reasonable compromise could exist (partial gift, clear written agreement, or helping with a limited, one-time contribution), but being railroaded into co-signing under pressure is a red flag.

Some will argue family comes first and small sacrifices are meaningful; others will say protecting your future is essential and parents should not strong-arm adult children into financial risk. Both perspectives hold emotional weight, but the unilateral nature of your parents' decision is the core issue.

Here’s how the community might see it:

“No one should be forced into liability. Protect your credit — this is a financial decision, not a moral one.”

“Help in a limited way if you can, but don’t co-sign. That’s asking too much and it’s manipulative.”

“I get wanting to support family, but your parents shouldn’t have sprung this on you last minute — that’s unfair.”

Consensus tends to favor protecting your financial future while encouraging alternative, transparent solutions that don't saddle you with long-term liability.

🌱 Final Thoughts

You earned your stability the hard way — it’s reasonable to refuse a legally risky co-sign that could derail your own home purchase. Parents can expect help, but they shouldn't create obligations behind your back or weaponize guilt tied to family dynamics.

If you want to keep bridges open, consider proposing a clear, limited alternative (a one-time gift, a written repayment plan, or a smaller contribution that doesn’t require co-signing). If they won't agree, standing firm is a healthy boundary that protects your future.

What do you think?
Would you co-sign in this situation, accept a smaller gift, or refuse outright to protect your mortgage prospects? Share your take below 👇