So I called my sister today, and I thought I will only being up the subject if she did. And... She did!
This is how the conversation went:
Sister: I heard you are going to center parcs, my two kids were invited, but as you didn't personally invite them they aren't going.
Me: ah right
Sister: why didn't they get invited?
Me: I initially invited mum and dad, they begged for my aunt to go, and therefore we had to get another room.
Sister: but why didn't they get invited?
Me: now I know for next time, that I need to invite them. But it goes both ways, whenever they go on holiday, they need to invite me to. And no one in this family invites me to their mini family holiday, but somehow if I go on holiday, I have to invite everyone.
Sister: my kids aren't everyone.
Me: but if I invite them,I have to invite my other 4 nephew and nieces.
Sister: but my kids have always gone with you on holiday, and they are closer to you than the others.
Me: well it's about being fair to everyone, and I can't just pick and choose. Also they haven't gone on every holiday.
Sister: I think they should have been invited.
Me: like I said, I know for next time. Which means they will now start inviting me in the holidays they keep going on, like when they went to Spain on their own, it's great to know I'll start getting an invite.
Sister: I will call you back in 1 minute
Call ends, sister calls back, but doesn't mention it anymore
Just to let the OPs know, I will not be going on a teen holiday with my nephew and niece, I just said that to clear a point. My sister had also just come back from mini family holiday, and I think she thought I was going to bring that up. Btw my sister has two children, but only my nephew was invited by my aunt.
Thank you OPs for helping me