except shelley is probably the most egocentric of all of them

And that's what makes them so interesting!
Et lux perpetua luceat eis

Thank you very much for your explanation!
Everyone in this family is so damaged and so egocentric that they can't think about anything else but themselves.
Poor Shell.

I think Shell feels that her family considers her a bit of a joke, not too smart, shallow and silly. She goes into this herself in the finale.
By choosing this particular song, she is literally describing herself in a completely different way. Every lyric is a different description of a character, changing, opposite and most importantly complex.
As her family watches her sing, the realization of their Mom as more than the caricature of a Jewish mother becomes apparentyou can see it on their faces (Maura as well).
Its a beautiful scene.
Et lux perpetua luceat eis