Did they refer to them as their step-moms? Their Dad's nth wife? Half-sibling's mom? What was the common terminology used by Mormons back in the day when they practiced polygamy for describing family relationships between mothers and their husband's children from other wives?
A partial answer (I don't know the complete answer yet):
Henry Eyring, famed chemist and father of President Henry B. Eyring, currently the second counselor in the Church's First Presidency, grew up in a polygamist family. His father was married to two women, who were sisters. According to the biography "Mormon Scientist", authored by Henry J. Eyring (Henry Eyring's grandson), Henry referred to his half-siblings' mother as "Aunt Emma" (see page 93-94 of the biography, published by Deseret Book).
This may seem trivial, to call your aunt "aunt", but I think it does show that, in at least this one case, the child's aunt-nephew relation took precedence in title than the half-siblings' mother relation.
Obviously this answer is only one case, and doesn't apply to cases where the mothers are not already related.