can you use sibh after caitheann also?
Yep. Only mé and muid/sinn changes it, and then only in some dialects. And, because this doesn't specify, sibh is acceptable in the translation. Though it's weird for multiple people to wear a dress.
I used sibh; it marked it wrong.
Technically, it should be be correct. But the semantics are a little weird, with multiple people wearing one dress.
It marked it correct for me
Then it must have been added by the course contributors.
This is not necessarily true. It would make perfect sense in the context of giving instruction between options. The boys wear the suit and you (plural) wear the dress. Using sibh should be correct.