jakean92 if it's on for a medical reason your mum should be fully aware of the situation and will talk to you about it if you nee to know.
I'm sure though, your 13 year old sister told mum not to tell her manly 21 year old brother about a potential urine problem or incontinence issue. However, if you raise the issue your mum will be open, i'm sure.
If however, your mum is none the wiser, then you have raised the issue with her, for your mum to get to the bottom of. It is not your situation to get involved in. You can mention the waste band, and you can mention the urine smell. She will then do some digging.
I think it's best left alone by you though, Jake. You don't want to go talking about your dipar kinks to your sister to find out she has a UTI.