Advice /Planning 1-Year Olds Birthday Dinner

I decided to host a small dinner for our immediate family for my son's 1st bday. (my parents - who are still married and flew in from out of state, my husband's parents who are bitterly divorced, my husband's new step mom, and my husband's dear grandmother). That's it. 8 adults and the baby. I told everyone the date and made reservations at a causal BBQ restaurant. I hear back from my step mother-in-law that she wants us to reachedule for a brunch or lunch that same day, or dinner another night because she has family in from out of town for a big dinner that same night for her mother's birthday. So I reschedule for lunch instead of dinner. Fine. I'm a little ticked off, but I do it. She says that's great and then asks if she can invite her mother and her brother in law and her sister who are in from out of town (who I frankly hardly know). Meanwhile, we haven't invited any of my own friends, any of my son's little playmate friends' parents, nor our own extended family (our own aunts/uncles/cousins) BC we really wanted to keep it small. (we are paying and don't have a lot of money and didn't want a lot of people to feel obligated to buy him gifts etc., just wanted something intimate). Now, IF I tell her NO, I know my father in law will sulk and be pissed off the whole time BC he is deeply whipped by this woman. Like he thinks: "what's three more people???? why couldnt they come????" He thinks I am very controlling. And I know he would've offered to pay the dinner bill, but I really wanted to host and be generous with our family. And there are just so many others that really deserve an invitation more than these 3 out of towners. My step mother-in-law will be really upset if she doesn't get her way. Lastly, my mother in law will be pissed and feel outnumbered socially surrounded by a bunch of non-family step-mother-in-law family members and will wonder why she didn't get to invite 3 of her own friends or family members. . . can't win in this situation. just wanted to do something simple. wwhyyyyyy does it have to be so annoying!!!????? advice? vent over.