Black cousins
Growing up as the youngest cousin, Regulus was shielded the most by his brother and cousins. He was closest with Narcissa and Sirius, but Bellatrix was the protector of the group. She stood up to their parents while all the others were too scared. When Andromeda ran away, Bellatrix distanced herself quite a bit. She still protected the others when she could but sometimes she acted like she couldn't see what was happening and she never helped them heal their bruises anymore.
When Sirius left, she withdrew completely. She treated Narcissa and Regulus as if they had betrayed her too, and sometimes she would help Walburga punish Regulus. (At this point she'd taken the Cruciatus Curse so many times that her mind was fracturing, and sometimes she had episodes where she didn't recognise her own family.) She still never physically hurt Narcissa, but she was cold and emotionally abusive.
Regulus had to face a boggart in his DADA class. (We don't actually know the DADA curriculum, as each teacher in the books had their own priorities, so realistically, Regulus could have learned boggarts in his fourth or fifth year). To his surprise, it had changed from what it was when he was a kid. (When they were young, their parents used to lock them in a room with a boggart as a punishment). It was no longer his parents, and now took the form of his oldest cousin.
Regulus knew his parents needed him. They might hurt him, but they would never kill him. Bellatrix had been his protector, but now he couldn't honestly say he trusted her not to kill him.