I'm going to ask my mum for advice about how to help my cheating sister
Is there really anything that can be done in this situation?
I've always been a good counselor for my sister when she was in need.
It may not be a good idea to go to your mom about your sister's private business. Your sister would probably prefer to ask your mom herself instead of having the two of you talk about her private life.
My mum is too old to give good advice to my sister. She needs someone youthful like myself.
That could be true but in that case, you don't need to ask your mom for advice. You would be betraying your sister's trust.