My dad married a woman 23 years younger than him, she's 11 months older than me. For many reasons, mainly things she's done and said to me or my children, I don't have a relationship with her at all. I see my dad but after a huge blow up with her about my children being "bastards" (even though I later married the father of my children) and me telling her at least I didn't leave my child half way around the world to marry a man twice my age while I was still married to someone else, he no longer brings her around when we visit. She hasn't been invited to my kids graduations, she wasn't invited to my wedding even though my father has been present for all of it.

I really don't want to pass judgement here, I don't know the entire story and I couldn't imagine having a step-parent that I went to school with. It sounds like she's just trying to be happy, and if he makes her happy, more power to her. She hasn't harmed you or your siblings in any way. I hope that you can continue to have a relationship with her. Good luck OP