Sinister did "no more mutants" before Wanda did it.
Except when he did it it was with the intention of as many people getting hurt as possible. And he wasn't sorry about it.
It actually really does matter in the context of the X-books. Right now they're willing to forgive anyone that's a mutant and are quick to judge anyone that isn't.
Except Sinister isn't a mutant and they're doing the exact opposite of judging him.
So basically it turns into "All mutants are standing together ... and this one guy that helps us." So do other people get a free pass for helping mutants? Not that we've seen so far. Dr. Strange did more for mutants than Sinister has and they turned him away. It makes the X-men come across as either hypocrites or idiots. Maybe both.