I cannot fully understand what you’re going through as I haven’t been in that position, but I know someone who went through something similar (unfaithful Dad who wanted an anullment before marrying his then-mistress). It’s not easy. The important thing to remember is that it in no way reflects on you or your “legitimacy”.
I can offer you a bit of perspective from the other side of things. My parents were both married and divorced before they married each other (in both their cases, however, it was the
other spouse who was guilty of infidelity). My dad got an annulment 6 or 7 years into my parents’ marriage (25 years ago). My mom just got hers earlier this year. I can’t tell you what my half-siblings think of the matter, but for me, I feel like a huge weight has been lifted off of me. For the first time in 30+ years, my parents are right with the Church. Their marriage is valid. They can receive the sacraments. It is truly a great joy for me.
I understand your feelings. You have every right to be hurt – even angry. However, looking at the positive side of things, this could be a chance for your father and step-mother to finally be right with the Church. This might be just the thing that will help your father get to heaven. Isn’t that something to be happy about?
I don’t mean to say that you
should feel happy right now. I just invite you to see it from another perspective.
I also want to say that, as far as I know, the Church will investigate the validity of
any marriage if someone comes to them requesting for it. It’s not a matter of “I can’t believe the Church is even considering it.” They consider it because someone has come to them asking them to consider it. They need to investigate it before they can pronounce on the validity of the marriage one way or the other. So your mother needn’t take it personally.