Well, Sucker Punch as you know is a Dark Type move, and thanks to the Psychic type, bad news for the Siblings. Of course, due to the lack of Fighting moves, there really isn't anything to counter that. The addition of Mach Punch would be of great benefit to the siblings, as would a Fire move against Steel Types.
That Psychic typing really sucks. If there was a Light Type to be introduced in the future, I'd think the Latis would be that, they're more reliant on Light anyway. But then again, that's only if they ever do introduce it...