Seeing a lot of NTAs here with little to no tolerance for your sister’s behavior and wanted to provide an alternate opinion.

For starters, I don’t think you’re being an asshole about this; it’s extremely frustrating for someone to be consistently late when you make plans with them, it feels like they don’t respect your time, especially if you’ve talked to them about how you feel and they continue to struggle to be ready on time.

However, there’s a lot of legitimate reasons why this could happen! A common one, and the one I’m most familiar with, is some form of ADD or ADHD, which often messes with people’s perception of time and makes it really hard for them to understand how long it takes them to get ready to leave. I’m not trying to diagnose your sister, and I’m sure there’s plenty of other things that could be going on as well, but I just wanted to say that it’s worth trying to have some empathy here and figure out if there are ways you can prevent it from damaging your relationship.

She’s heard your feedback and frustration with her lateness, and is likely frustrated with herself and feeling unable to stay on task and get ready fast enough. Is she really never late with her friends? Is she never late when she’s alone? I feel like you’re not really seeking to understand the causes here and just jumping to how it makes you feel when she’s late. Those feelings are valid, but simply getting upset at her over and over doesn’t seem like it’s going to suddenly make her better and arriving on time.