I think it's probably very realistic; there does usually seem to be one totally irrational parent.
The acting is excellent which is probably why the mum (brilliant turn from Sharon Horgan) irritated the hell out of me. Michael Sheen really great but, with more equivocal views, was not 'allowed' to express them and pretty much steamrollered. The scant attention on older sister reflected well the actual lack of attention she (as for most in that situation) had to endure and 'smile' through.