There is a passage in DH chapter 33 'the Prince's Tale' that shows James, Sirius, Snape and Lily on the train.originally posted by dancing_in_moon_pools
Who said his opinions are set when he reaches Hogwarts?
maybe he's like not sure, and is angry when hes sorted into Gryffindor, but then comes to love it and his friends.
Regulus probably saw how Sirius was punished and then firmly made his mind up to not be outcast like Sirius. And Regulus could have been the type when he was younger to believe anything he was told.
The fact that Sirius didn't smile until he said about breaking tradition sounds to me as if he didn't want to be in Slytherin."Who wants to be in Slytherin? I think I'd leave, wouldn't you?" James asked the boy lounging on the seats opposite him, and with a jolt, Harry realized that it was Sirius. Sirius didn't smile.
"My whole family has been in Slytherin," he said.
"Blimey," said James, "and I thought you seemed all right."
"Maybe I'll break the tradition. Where are you heading, if you've got the choice?"...
Regulus would have seen the commotion caused by Sirius not being in Slytherin. Sirius decorated his room in Gryffindor colors and banners, Regulus decorated his in Slytherin.
I think Regulus enjoyed the fact he was sorted the same as his family.
I also think he joined the Death Eaters of his own free will and choice. Was this choice based on his upbringing. Yes, I think it was. I am sure he was preached to about pureblood status his entire life. And the fact Sirius was not sorted into Slytherin probably set off their mother to no end. In the books, we see how nasty her portrait is, is there any reason to believe she was not like that in real life?
As fas as Sirius and Regulus being close. I think they were until after Sirius started school, there is only a year or so between them in ages. I am sure when he came home the first time after he was sorted, it was not a pleasant time. I think once his mother found out he was in Gryffindor, she started preaching to Regulus how horrible Sirius must be. How he wasn't pureblood enough to get into Slytherin.
Sirius probably got a lot of attention being the oldest and the heir apparent. So, once their parents turned on him and focused on Regulus, I think Regulus enjoyed getting the attention of his parents and decided to mimic their beliefs.
The parental disapproval of Sirius, I think played an important factor in the way Regulus turned out.
Though at the end, he probably realised Sirius was right and his parents were wrong. I think this is what caused him to turn against Voldemort. But his love for his parents kept him from openly renouncing Voldemort. He kept quiet to keep them safe.
Just my 2 Knuts worth to add to this interesting discussion.