Feeling a bit fed up with Step Daughter's Mum and Step Dad

I'm feeling so fed up and just need a place to vent. I have a newborn and a teenage step daughter. Me and her Dad have her every weekend, Friday night to Sunday evening. He drives a two hour round trip both Friday and Sunday so she can stay with us each weekend. He pays child support to her Mum and we have her pretty much every bank holiday and extended weekends in the holidays. Today my partner had an accident at work where he hit his head pretty badly and has a huge lump on his forehead. He, understandably doesn't feel great. I questioned if he was safe to drive for two hours. He was unsure so checked in with SDs Mum to see if they had plans or if he could pick her up in the morning when it would be safer. They had plans so he decided to go tonight. But before he could message her mum back, her stepdad text him accusing him of being flakey. I'm so mad. I'm so sick of her stepdad acting like my partner is a deadbeat when he is nothing of the sort. The stepdad gets mad that my partner can't look after my SD for the whole summer holiday but he is working full time. Its either he works full time and her mum gets the money or he doesn't and he can't afford to pay her. And now my partner is driving for hours with a possible concussion because her stepdad is being an ass about it. Its so frustrating.