What would be the politest way to introduce my dad's wife to people?

My dad married Jane in 1981. She’s only 8 years older than me, which makes her my husband’s age. My dad passed away in 2004. Jane is coming to visit us. I don’t know how to introduce her. I’m not the least bit comfortable calling her any form of “mother,” considering she could easily be an older sister. None my sisters have ever considered her any kind of mother, either. She’s just “Jane.”

My kids, however, have grown up always calling her “Gramma Jane.” Everyone is comfortable with that.

I always call her “My dad’s wife,” as I did in the question, but I’ve never had to actually introduce her to someone. I don’t know how she’d feel if I introduced her that way.

Any suggestions?