Why did she sing???

She doesn't.

wdym, https://www.google.com/imgres?q=max%20and%20lucas%20singing%20neverending%20story&imgurl=https%3A%2F%2Fi.ytimg.com%2Fvi%2Fud56EL9dt5Y%2Fmqdefault.jpg&imgrefurl=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3Dud56EL9dt5Y&docid=wzRt-NQpaEgTrM&tbnid=2ZE4nkLEs7URfM&vet=12ahUKEwjv06jFwKqWAxVZkyYFHaEXPTsQnPAOegQIPxAA..i&w=320&h=180&hcb=2&ved=2ahUKEwjv06jFwKqWAxVZkyYFHaEXPTsQnPAOegQIPxAA

This happened right after billy died