OP, it's obvious to me that she wants someone to share the cost of her children's party.
She's probably seen the cost and thought, "We should share with someone." I don't know why she's thought of you and not someone else with a birthday around the same season, but she's picked on you.
For one thing, why would your one child want to share a party with two siblings?
For another, why, given your child's birthday is two months ago, would you want to have another celebration now?
You have to think why she asked you. Are you usually pretty meek in her presence? Does she think she can bully you into it?
You just need to text back, "But DC had her birthday party in June! Your DC (name) came to the party!"
For god's sake don't go along with her manipulation, OP.