AITA for wanting my sister to pay me back for som**hing I bought her months ago

As renowned researcher Dr. Brené Brown explains, “Boundaries are the distance at which I can love you and me simultaneously.”

This situation centers on a failure of transactional integrity within a family dynamic, complicated by the nature of the purchase itself. The OP initially set a clear boundary: the favor (buying the item) was conditional upon repayment. The sister accepted this condition. Her subsequent delay, attributed to an unwillingness to exert minimal effort (going to the bank), demonstrates a lack of respect for the OP's stated financial constraints and the agreed-upon terms. While the amount ($30) is minor, the principle of honoring a commitment is significant, especially as the OP is transitioning to college where financial responsibility is critical.

The OP's emotional response—feeling sick and tired of waiting—is a natural reaction when perceived support is met with non-compliance. However, threatening to involve the mother crosses a boundary into potentially punitive action, leveraging shame to enforce a financial agreement. A more constructive approach would involve one final, clear communication of the boundary, perhaps by stating a firm deadline for repayment before taking further action. If repayment is still not made, the OP should enforce the consequence regarding future favors without involving the parent, thereby maintaining personal agency over their boundaries without resorting to relational blackmail.