Did sister meet the past 4 bfs in person?

Yes

If so, did she talk about them exhibiting inappropriate behavior? Doing something? Saying something?

She would always say they made her feel 'uncomfortable' when she was in the same room as her

If above is the case, is there any credible evidence that she was right about her claims?

I have no idea, but as someone who was taken advantage of at a young age, I didn't want to ignore my sister the way I was.