Finding out the sex decisions to be made...

Hey guys. I have a tricky situation that just came up and I'm not sure how to go about it. We made it through our first trimester and are a little over 13 weeks. We had our ultrasound today and things look great....huge milestone for us since we have had two miscarriages and gone through a lot of tests. We scheduled our 20 week anatomy scan as well where we will find out the sex. I told my mom when we will find out because it's exciting. My husband and I have talked about how fun a sex reveal party would be to do with our families but we weren't sure if we wanted to find out ahead of time. My mom was totally on board. We haven't decided anything yet though because we are still enjoying the moment of making it through the first trimester. My mom knows that my husband and I are extremely close and how important this is. She out of nowhere told me my sister in law would take off work to babysit my nephew (my mom takes care of him while my SIL is at work) so she could come to our 20 week appointment to find out the sex. I'm a little confused as we never discussed her coming along to the appointment at all and she also knows we haven't decided if we want to do a party or want to be surprised or what. So I told her we weren't sure what we wanted to do yet and I had wanted to surprise her by doing something cute. She got upset with me and told me she won't wait to find out and if she doesn't go then she wants to know right after the appointment and doesn't really seem on board with doing a reveal anymore. Part of me is irritated that she is acting this way since it isn't really about her. Part of me is confused as this kind of came out of the blue. Part of me also feels bad because I know she is very excited that we are having a baby and wants to know the sex as soon as possible. My husband says we shouldn't have to accommodate other people. We have bent over backwards to accommodate both of our families throughout our struggles in one way or another and he says he is tired of it and we won't do it this time. I agree. I totally do. But at the same time, she is my mom and I love her and don't want to hurt her feelings. Have you guys been through something like this or something similar? How would you go about dealing with this? Thanks for reading and thanks for your opinions and ideas😊