Dear new step:  you don't recede into the background.  You welcome the girl into your family activities.  Instead of being jealous, plan for her visit with little gifts, gathering craft supplies, or fixing up her room.  Be sure to allow time for them to be one-on-one as well.  Ask your husband if you can plan a family outing for all of you.  Treat her like a favorite niece and allow her time to get to know you.  Hopefully she will want a relationship with you, not quite MOM, more than friend, and as someone who loves her. And be nice to her mom (as much as possible, if mom is receptive) so gatherings can be pleasant.  You can set the tone....decide if it's going to be loving and accepting, or territorial and cool.