@GracieGirl,
Gracie, one never knows what goes on in a relationship between 2 people and your mom and dad might have had an agreement, who knows! I am sure your dad is ready to talk about your mom once he feels you're old enough to understand.
It is the same with my daughter (we call her Jane here), when she's 18 years old she'll able to search for her birth mother, until then I think she's too young to understand certain things. No, I have no hurt feelings at all, I know Jane loves me and I always will be her mom, but I also understand that she wants to know about her birth parents and the circumstances why she was given up for adoption. I would want to know too, so it's only natural for Jane, and yes, I will help her - this is a big part of her life and I would not want her to do this in secret and not being able to talk to me about it, while thinking I do not approve. No, not at all! I don't feel threatened in her searching for her birth mother/father, on the contrary.
Life is not simply black or white, it comes in so many different shades and
we have to embrace that and adapt accordingly. You're still a bit young, Gracie - even though you're more mature than most 13 year olds - but when the time comes, your father will open up too! Trust me, your mom loves you, your sister and your brother - no question about it! Don't doubt that for a moment!