Jennifer Lopez and TLC have done the same.
You can hear Christina Milan all over "Play" by Lopez
and there's also a song of hers where you can clearly hear Lil Mo singing.
Debra Killings has done several background vocals for TLC
Janet sung background vocals on PYT.
Toni Braxton has utilized her sisters often.

It's not unheard of.
If its not excessive, and the artist is singing the leads I can let it slide lol.