AITA for not getting over my sister “borrowing” money?

I (17f) inherited $6,000 from my great grandfather a few months ago. Since I am under the age of 18, the account was a custodial account which means I have to have someone to take care of it. This means that I cannot take money out of the account without the permission of my custodian but the custodian can take out as much as they want. My parents died when I was 7 and grandparents became my guardians. They’re my father’s parents while my great grandfather was on my mother’s side so they were not in the will. The lawyer that set everything up said it would be easier if someone in the will was the custodian so my sister (26f) became my custodian. My sister also inherited $6,000 that she quickly spent. She has two kids (6f and 5m) and her and her husband are both unemployed. When I set up my account, I got $1,000 out so there was supposed to be $5,000 left in the account. My sister has a record of not being good with money so me and my grandparents agreed it would be better to get the money out of the account and put it in one under my name. After weeks of asking my sister to get my money out, last week she gave me $1,000 of it and said that the bank wouldn’t allow her to get anymore out at one time. Yesterday, I got a letter from the bank telling me my bank balance. She had gotten $870 out of my account without telling me. She would not pick up my calls so I had to call her husband to talk to her. She started yelling about how the bank must have messed up and took the money out of my account instead of hers. She said that she would get the rest out of the bank and bring it to me and that she would pay me back. She got the rest of the money out today and there’s only $1,800. There was supposed to be $3,135. She lied about the $870 and then got another $1,335 out for herself. She owes me $2,185. She said that she is going to try to get a loan and if she can’t she’ll pay me back using her tax returns. She wrote a letter about how she knew she was in the wrong and that she had meant to get the money back in the account before I noticed. I am mad at her but our other sister (24f) thinks that I am being too hard on her because “I’ve never known what it was like to struggle for money like that.” My grandparents are mad but my grandpa has a weak heart and wants to put this all behind us because the stress is bad for his health. My family thinks that I need to be more understanding of her circumstances and to calm down because of my grandpa. I don’t know if I’m making it too much of a big deal but I need that money for college. I got a scholarship for full tuition and I qualify for a pell grant but everything that the scholarship or pell grant doesn’t cover I was planning to use the money for. My sister says that it was obvious she was going to pay the money back because she would “never jeopardize my chance at college.” Everyone thinks that I should get over it because she said she’s going to pay me back. AITA for not wanting to just “let it go”?