*Nonni goes to look for Webster wetnurse definition.*
A wet nurse is a woman used to breast feed and care for another's child. Wet nurses are used when the mother is unable or chooses not to nurse the child herself.
I suppose it could also be considered a new occupation?
I'll see what I can do. I think it's quite possible to only circumvent the check for family ties if the player ORDERS someone to nurse a baby, but keep the check in place when a sim tries it autonomously. Quite likely, this would allow ANY female (maybe even teens) to nurse ANY baby, as long as the player tells them to.