Long story short me and H are separating and discussing contact arrangements with our DC. He has DC from a previous relationship.
My husband's older DC have no set contact arrangement. It changes week on week depending on exes shifts for that week. My stbeh wants our DC to stay on the nights that DSC stay so they can see each other.
This would mean their contact would always be changing as well based around their siblings parents work shifts 
He thinks this is best because it allows them to continue their sibling relationship undisturbed which I appreciate but it also means our lives continuing to be dictated by this ever changing schedule and I don't want that.
AIBU to insist he should have set days for our DC and not really care whether that falls on the same days he has his older children? It doesn't actually affect me much work wise either way but I'd feel like I was still enthralled in it all and I want to be able to plan out my week appropriately.
YABU - sibling contact is more important so your DCs contact should be flexible too.
YANBU - contact with our DC should be fixed regardless of exes older DC.