Yes I'm afraid you have to really spell it out clearly to these very cheeky people. I am sure most of us are willing to accommodate a genuine emergency in terms of child-care but it was wrong of her to put you on the spot like that.
And her reasoning is ridiculous; all child have to learn at some point that there are things they are invited to and things they are not invited too! And if this little boy is genuinely disappointed (and she doesn't just fancy an afternoon shopping) it is his parents job to help him manage his jealousy and upset; not you!
Besides, an older boy could totally change/upset the dynamic of the party!
Send her a text immediately and tell her you have no room for siblings now you have had more replies! Do not apologise!
Sympathies though; you sound lovely op to be worrying about this, and there are sadly lots of pushy people about.
I know a similar mother who can only see things in terms of her own children's viewpoints and interests to the exclusion if everyone else, including other DC. In her case it comes from an intense, somewhat blinkered tunnel vision of love for her DC, so I let her get away with it most of the time, and because, well, because life is too short! In other cases though, people are just taking the Michael!
Have courage op and hold your ground!