okay drama queen.

You would know who your mother is. Don’t obsess over this. Yes it’s weird and embarrassing, but those things exist in life.

Keep it in the family!

This... this doesn't strike me as all that weird or bad. Your aunt and dad went on one date in high school [Or, perhaps on what you specifically mean by "once dated", they might have dated for a little while], decided that they didn't fit together and found new partners. It's not like your dad ditched your aunt specifically to hook up with your mom or anything, by the sounds of it. Sometimes people do find new partners within the groups of people they already know. And if your aunt and dad had a serious, long-term relationship, it certainly would be more likely that your mom wouldn't have dated him as well, to avoid potential awkwardness. But the fact that you didn't know about this until just now suggests to me that there's not like, any major family-friction about it, meaning that it wasn't a big deal, so probably not a case of dating a long-term ex's sibling.

Chillax

Perfectly normal. She dated your Dad but married your Uncle. This has zero effect on your family tree.

As long as your aunt isn't your dad's sister then it's fine.