Olivia
I would prefer to spare Cygnus the responsibility of being heir and for Oberon to get his act together. That's going to require the Lestranges to show him some tough love, make him get treatment and make him get a job so that he's not home wallowing in his misery all day.
I wonder what happens when they're all too distracted to babysit? I think it might be just the push that's needed for some changes to be made.