AITA for refusing to pay for my sister’s bday present (home renovation) thus breaking my promise?

So this is a hard one. I can see how I’m the asshole but looking for more feedback.

My younger sister and I have always been close growing up but in adulthood there’s been more tension. Namely the fact that my husband and I earn a lot more than her and her husband. It’s not a huge deal but it’s a sore spot for her and my parents. They think I need to contribute more to family but I think there should be limits.

Anyways, sister recently bought a new house and my mom suggested I buy her a house warming present that’s like a small renovation. I agreed and we ended up settling on me paying for small renovations of one downstairs half bathroom. In their words, this meant retiling, new sink, new toilet, etc. I was fine with this.

Well the construction finishes and the bill is 4x what I expected based off the items they told me. They actually EXPANDED the half bath to a whole bath, with shower and everything. I asked a contractor friend and he said this was unprecedented and also illogical (who showers in a ground floor guest bathroom?) They also tore up half the house to make this happen. They chose the most expensive items possible (like shipped from overseas) to rack up the bill.

Anyways I saw it and said I’m paying for half of this shit, which is already more than what I had expected. My mom and sister flipped their shit saying that I’m reneging on my present and that I should’ve put a monetary limit on my gift if I was planning on being so stingy. They said since I didn’t I owe them the full amount.

Are they assholes or AITA?

Edit; also to add this was a combined present, housewarming and also sister’s upcoming bday in May, which my mom thinks warrants a huge gift. Final cost was around 13k to give a sense.