Don't really know if this is an etiquette question or not, but here we go.
My fiancée and I are just trying to write up our guest list, and we are having trouble decided weather we should invite my step-mother or not. She and my dad only divorced 2 years ago, and she was with my dad for 17 years, so I love her, and she was part of my life for a long time, and I still speak to her as often as my dad. She has a boyfriend, so I'd be inviting him as well. My fiancée and I really want to invite them since I still see her as part of my family, but I'm worried that its inappropriate. i also know for a fact that my dad and his girlfriend won't be civil with her and her boyfriend, since he still has trouble being civil with my mum and step-father, and my parents have been divorced for 23 years.