I highly doubt that OPs sister was alone in making a decision to live there. OP had to accept the deal otherwise her sister wouldn’t be there. I’m simply stating that if this was a leased agreement, then OP would be in the wrong here. Take away the lease and the premise remains. Threatening to kick out your sister mid semester whom you agreed to house for what now probably feels like to OPs sister a nebulous set of requirements.

I say she’s TA because I’ve been in her sister’s shoes before. In college I had my own car. It was small and couldn’t fit my bike inside so I was considering buying a truck instead so I could go mountain biking and camping and such. My mom wanted me to give that car to my brother who was starting to drive and said that she would buy me a truck I wanted. It wasn’t super expensive, not more than the vehicle she would’ve purchased my brother. But she held that over me ALL the time.

“You know I should’ve never bought you that truck if you weren’t willing to do something nice for me” and the “something nice” was always petty and ridiculous. Like I had my own vehicle, you offered to pay, and now you’re holding it against me?? No. Lmao.

I paid her back for it because I was sick of it.