I think having it be the sister is significantly different from being either parent. If it was a parent or older relitive, I'd personally lean more towards no.
I think that the best way to go about this would be to pose the same question to M and see how she feels, she may not be comfortable meeting some so close to you so soon. If the roles were reversed what would you want M to do?
If it were me, I'd bring it up to M and let her decide. If nothing else is shows that you have a good relationship with your sister and you have enough respect for both of them to introduce them on a professional basis.