Being a step mom?

So I am engaged and have been for almost a year and my fiancé has children.. 2 to be exact and the woman he has children with refuses to communicate and it’s starting to piss me off. My bonus daughter just had a birthday Tuesday where she turned 6, and we were trying to do a joint birthday party instead of having two Separate ones and so we agreed to pay half for the birthday party and she was to pay for the deposit but when asked 3 days prior she said she couldn’t pay it and excepted us to do and we refused to pay for everything because we pay a lot of money for everything the kids need.. it was supposed to be split so as they talked she became really rude with name calling etc and of course I got offended because that’s my fiancé and I know what type of father he is to his children but I never say anything because we aren’t married yet.. but moving on I decided to pay the deposit Bessie she said she was going to do something different for her and me and him need to do something ourself so we booked and of course today she decided to take her and make us look like the bad people im just sick of it and idk what to say or if it’s even my place. Has anyone had to deal with a woman like this? What am I supposed to do?