Family is really mad because we missed grandmas birthday?

So on Sunday my mom mentioned she’s having a get together for my step grandma on tues.. I said wonderful let us know what time on Tuesday.. Monday night I ask her, what time is the party tomorrow ? She says I’ll let you know I have to think about it.. well I worked all day and didn’t hear a word.. at 5:30pm she asks me what I want for dinner.. I’m like for what ?... and then she’s like for step grandmas birthday.. I’m like it’s still happening?... she says yes it’s at 6:00... my eyes get huge I’m like what?! Why didn’t you tell us earlier.. we live an hour and a half away... I said oh well we can be there at 7:30.. (which would mean I wouldn’t even get to shower or get the baby’s stuff together) it would be 7:30 if we got in the car and headed that way now! They said oh that’s too late don’t bother.. and are now mad at us.. got a nasty text from step grandma.. parents called us selfish and said we planned this the whole time etc.. just NUTS... Are we crazy? I mean.. for real..they tell us at 5:30 to be at an event that’s an hour and a half drive.. that starts at 6pm .. now we are the people that didn’t attend grandmas birthday.. my step dad texted me saying they won’t forget this etc all mad.. like what in the hell..? I feel sick to my stomach and quite frankly horrible for the whole thing! I told grandma it was a mixup and she didn’t even respond