You are probably over reacting just a little, but yes, it's human to be a little hurt, so don't worry about that part.
What she's doing is common enough practice. Look ahead - how would you feel if the house you're in, that you spent your family life in, saw your kids grow up in, loved in and possibly died in, was lost to a divorced ex-partner of one of your kids? Although in your case this will never happen, she's just looking after her son. Try to see that for what it is, and not take offence, she's just doing her best for her family.