OP rarely gets to see her mother, sister or nephew because they live a few hours away from them, presumably to facilitate OP's husband being close to his older DC. If OP herself rarely sees them, then how much more rare must it be for her step DC, who are only there weekends? to have met or got to know OP's family. They're pretty much strangers to OP's mum, sister and nephew, and weren't supposed to be at OP's on the date this outing was arranged for, which is WHY it was arranged for that day. OP made the choice to marry a man with Dc's and take them on like her own, but her family didn't. OP doesn't get to dictate to them that they must see or treat her step DC like her blood DC. It is perfectly reasonable for OPs mum and sister to want this to be a day out/catch up with just OP and their bio grandchild/nephew, especially as it was arranged/paid for by OP's mum, and not want to invite Op's step DCs. It is neither Op's or her Dh's place to invite Dh and the step DC's.
Op's updates make it clear step DC don't get on with eachother and are full on and argumentative, and that her DH won't discipline them for their bad behaviours as he doesn't want to be the bad guy to his kids. It was also made clear, despite many people still suggesting it, that her Dh simply would not take his 2 older DC around separately, as he does not want to be in sole charge of them, even when at home. OP stated he has always refused to do this previously, so she knows he wouldn't. He will insist on staying together as one group, and not discipline his badly behaved kids, who would probably try to dominate the day, no doubt spoiling it for OP, her mum, her sister, and her nephew. (Tbh, i wouldn't put it past him to announce on the day he "can't take the time from work" and not go along to help at all)
Everyone is getting so up in arms about how it's cruel and unfair to the 2 step DC, well, what about ops DC and her nephew? Why is it more fair for their day out to be ruined? Why is it ok for OPs, her mums, and her sisters rare day together, to be ruined? Why do 2 kids and their feelings, outrank 5 other people (2 of whom are children also) and their feelings?
The step DC could have an awesome, fun day out somewhere else with their dad, and not feel at all left out. But HE is the one being selfish and not wanting to do that, and has never done it previously, because he doesn't want the responsibility, or feels he can't cope with them alone (which makes it even more unreasonable to expect someone who isnt related to them/in a parenting role to do it instead) The blame for any hurt feelings is 100% on his head, he has the oppertunity to create a solution where everyone is happy, but he's too selfish.