My MIL likes to hijack my little boy's b-day. Or am I being too sensitive?

You actually Agreed to let her throw a larger birthday party - But She was Wrong because she Didn't let You know When it would be in advance. I Don't believe for One minute that "party" Wasn't planned in advance. (Why would she have a birthday cake And All those guests if Wasn't planned? That many peopke Don't just show up All at Once! And if it Wasn't planned, I assume there were No gifts then, right? Ybesides hers.) I didn't think so!) Let her know you are planning his One & Only birthday party this year & try Not to let him be with her the week Before until the week After his birthday - Good Luck with this one!