When your seven year old half-sisters ask you this, how do you even come back from that?!Were you trying to be normal?
I really don't think that "when you're asked for wishes, answering you don't want anything can sound offensive to the one asking" is an idea too complex for a seven-year-old.The most specific line would be telling Hotaka that Hotaka not requesting anything would actually hurt Wahei's feelings when he's trying to build a better home, instead of making Wahei think that Hotaka not having any wishes would be a sign of an already perfect home. That's quite abstract thinking from such tiny kids.