I'm actually looking forward to her coming back. I agree with those who say that the actress was quite limited and that did have a negative effect on the character at times, but maybe she just was too young at the time, and has improved since.
In general, I am not a fan of the "family member leaves never to be seen or mentioned again even though they supposedly live close by and used to be really close". I understand that actors want to try other things, so it's unavoidable in many cases, but it just feels so unnatural.
I would have loved the sisters to come to Adam's wedding. I know it wasn't planned, but it would have been a nice touch to show that he has family beyond Moira. And Moira herself really could do with someone female backup in the upcoming months, judging by what we have seen from Charity and Emma. Especially if Cain really ends up leaving her (not a spoiler, just speculation), it could leave Moira quite isolated as a character.