What will happen when caught using a 11-15 oyster

Possibly both.

As poster above said, possibly both. If this is the first time, you'll probably just get a fine. Don't do it again though - TfL will keep you on file and they won't be kind to you if you dodge a fare again.

And next time you lose the Oyster card, use contactless.

Yh I’ve learnt my lesson, what will happen to my sisters oyster will it be returned or will she be banned for a certain amount of time

This document might be of use to you/your sister:

https://content.tfl.gov.uk/zip-enforcement-policy-new.pdf

Alright thank you I’m just praying I only pay a fine and not court I might have to make an appeal if need be to either settle outside of court