I would normally agree about a praise for a family member, but when a Presidential candidate says someone would be a “phenomenal Supreme Court justice” I think it is relevant, family member or not.
I would generally agree about saying that about a judge, but Trump would obviously never nominate his sister to the Supreme Court (again, she's way too old, she's his sister, and in any event, he's not going to be President anyway). So, given that, I view it as a pretty inconsequential statement that was meant to be nice about his sister...