While Luke is treated as the Black Sheep by almost all of his siblings (which, admittedly, isn't entirely uncalled for, considering he himself has admitted he screwed them over numerous times), his twin sister Nell is the only one that doesn't give up on him even when he has her purchase drugs for him one last time while they are on their way to an addiction treatment center. He shows similar care to his friend in rehab, Joey, and sacrifices his own attempts at recovery to help her when she flees the center.
Nell and Arthur's love montage is very sweet. It's heartening to see someone wholeheartedly support Nell after all she's been through. Unfortunately, they're Too Happy to Live.
Nell and Arthur's wedding is also one of the relatively bright spots in the family's history:
Steve and Nell finding out Theo is gay when she sleeps with Nell's maid of honor in the middle of the reception and their only response is to affectionately hug her.
Steve and Nell dancing, which is one of the only times the audience sees him act like a proper older brother.
Hugh telling Steve about the time he and Olivia were separated for two weeks and then got back together as An Aesop for how he hopes Steve and Leigh can get back together. Notably the normally snarky Steve is quiet and thoughtful throughout Hugh's story.
Ghost Nell telling her siblings that she will always love them no matter what.
The remaining Crain siblings finally being able to fix their lives after escaping Hill House. Shirley reconciles with her husband and tells him about the affair she had, Steve fixes things with Leigh and is able to let go of his fears of fatherhood, Luke becomes clean and sober, and Theo finally moves out of Shirley's guesthouse and presumably in with Trish, even finally taking her gloves off. The last we see of them is them being happy together and celebrating Luke's sobriety milestone.
