In a blended family, “That’s not fair” often means “I don’t feel seen.” Answer that part too.

Fairness arguments in a rearranged household are rarely about the size of the portion or who got the bigger room. Counting is just the only available language for a much harder question, which is whether you still matter here. Fix the arithmetic and the complaint returns next week attached to something else, because the real question went unanswered.

When somebody says it, settle the practical bit if you can, then ask what else is going on. If you are the one saying it, try naming the second part out loud instead. It is a harder sentence and a much shorter conversation.