What do you do in situations of step parents? How involved were they in the wedding party?

My fiance's dad married his now wife later in life. (His 3rd wife). My fiance did not grow up with her, he met her in his late teens and doesn't refer to her as his step mom, only "his dad's wife". She has adult children of her own and his dad has my fiance. So, they have no children together. We all spend time together at their house (dad, dad's wife, her adult kids, fiance and I). I want to ask her to go dress shopping with me and my mom, aunts, and fiance's mom. (Fiance's mom and dad's wife get along). I think it would mean a lot to his dad if I asked his wife to come too. She has a terminal illness and only has a predicted few years left. My question is, how do I navigate further with this relationship in terms of her involvement of wedding party? She isn't referred to as his step mom in every day life so would she be introduced as that? Would she get a "Mother of the groom" dress along with fiance's mom? Would my fiance also share a mother of the groom dance with her? Does anyone have a situation similar to this? What did you do? via /r/weddingplanning https://ift.tt/2Vzfu4B