Belting is supposed to be brassy, my voiceteacher once said.
I don't think you can define it with the word pretty, because it is an opinion based adjective that makes an argument like the one it seems you are having with your family a bit hard to end / continue into anything other than name calling or off-topic comments.
Try using other adjectives like strong, dainty / delicate, or whatever else comes to mind. Maybe talk about the situations / songs that they think are pretty and see if that helps with the argument?
Hope that helps!
"If you can talk, you can sing...if you can walk, you can dance."
- T.K. Greene