AITA for not asking my grandparents to include the other kids in my house when we visit the water park?

I (16M) have a really crazy house because my mom remarried after my dad died. Her husband already had two kids when they met who are now 12 and 11. Her husband's best friend went to jail 18 months after my mom got married to her husband and they took in his best friends kids who are now 8 and 6. Then her husband's sister died and she had a a kid now 5 who also came to live here. So in total there are 6 kids (including me) in the house and two adults.

My dad's parents see me a few times a year. My mom doesn't let me see them too often because she's worried the other kids will feel left out, especially because my grandparents don't automatically include them or send them gifts and stuff.

About a week ago they told me they were planning to take me to this water park that got renovated about 2 hours away from where I live. They said we could make a day of it and mom already gave them permission to take me for the whole day. Mom was listening to my conversation with them and right after the call ended she asked me if that was the place the other kids had wanted to visit. I told her I wasn't sure but probably since everyone was talking about it at school.

My mom made a noise and then a few hours later she told me she had talked to her husband and they wanted me to ask my grandparents to include the other kids that day. She said they could never afford something like that but my grandparents can and she's happy to pay to come along and be an extra set of hands. But she feels like they should be willing to include all the kids this once because it's something so popular and exciting.

I told mom I didn't want them to come with us and I wanted to spend time with my grandparents. She told me I needed to ask because they would never say yes to her but if they think I want them there it'll convince them. She gave me a few days to ask and I didn't and then her husband brought himself into the conversation and the two of them have reprimanded me for being selfish and for being greedy. They said I have the chance to do something kind and wonderful for my "siblings" and it's not like mom isn't willing to come so she can help. But I need to accept that I have the only grandparents in this family and that means I should be encouraging more of an inclusive dynamic between us all. My mom told me she might not be able to say no to it but she can remind me that I'm already not the best oldest brother in the world and one day I might actually think of them as siblings and regret not trying to make them happier.

AITA?