Can I add a family member to my apple one premier if they already are family sharing with their son? What will happen if I try?

I have successfully shared apple one premier with a family member, but I have another family member I have not added, because she is concerned that this might cause a problem with her existing set up. Currently, she has her own apple music and TV subs, that she shares via family sharing with her 13 year old son. I wanted to add her apple ID, but not his Apple ID, to my apple one premier family sharing.

Will this even work, and if so, how will her sharing situation be affected?