Mother In Law invited strangers to our wedding

Hello everyone! I do not know how to handle this situation, so I am hoping you’ll be able to offer some advice. My mother in law has a daughter from a previous marriage that my fiancé only found out about at age 20, he is 23 now. My fiancé and his younger brother (21) have never met their half sister. Yesterday we called Fiancé’s mom to inform her that we set a date. She was very frazzled and not happy about it at the time, kept bringing up a million reasons why we should postpone. (Our wedding is in late October 2021) One of her reasons for needing us to postpone was that she was picking up his half sister from the airport for a friends wedding in October. This surprised us both, because we are pretty certain MIL and half sister have not seen each-other in person, at least in the last 10 years. When he heard about the sister being in the area at the time of the wedding, my fiancé off handily said “should we invite her or something?” To which his mom ignored and continued to talk about her grievances with our wedding date. She said at the end of our conversation that she would tell my fiancé’s father the news and was hoping he would not be upset with our date.


The next morning she calls, and she’s a a very chipper mood. She tells us that my father in law is happy to hear the news and she finally congratulates us. The mood shift was welcome until she informed us that she had already informed his half sister (that none of us have ever met) that we are getting married, and that she will be taking off extra leave from work to come to our wedding after her friends. She tells us her and her husbands names (that we have never even heard before) and gives us their address, telling us to send an invitation. She also informs us of some family friends she wants invited, and says we will be rude not to invite them.
Our wedding has only 40 people on the guest list. They are family only, not even a single friend. I have always wanted a very intimate wedding, and this seems safest considering there’s a global pandemic going on, and no one knows where we will be in October. My mother in law has has issues with boundaries and manners in the past. How can we tell her that we are not inviting these people just because she says so? I have 3 half siblings myself that I met as a very young child, and I know the time and place to meet them for the first time is not your wedding!
How can my fiancé and I handle this without creating drama? Any advice is appreciated. Thank you for reading.