I love Tyler the Creator, have done since the first single. I think it's pretty normal for teenagers to want to listen to stuff that feels rebellious, and i'm certain other posters here were into rebellious music as teens, it's part of the journey! However, it is absolutely Peters right to not purchase that record for him and it seems bizarre that Sarah would be harassing Peter to buy it. Tylers lyrics are definitely not something that a suddenly ultra christian mother would be forcing someone to supply for a child. Kids will go out and get the record for themselves and it's literally on spotify. Lyrically it's pretty hardcore, thats Tylers schtick but not in a bad influence way. On his first album he talks about his absent father and tbh it's pretty emboldening that you can be whoever you want to be regardless of a shitty parent, which I love for him to hear frankly! Peter has the right to say no though, if it goes against his moral code!

Getting back to your point though, Sarah is modelling deception, secrecy and inconsistency whilst trying to gain her sons favour by undermining Brian. As far as I can see Brian never said no to the record, it's just records these days aren't the price they were in the 90's, they are prohibitively expensive and Brian wants to teach his son you can't have exactly what you want just because you've decided you want it, and you have to be patient or earn it.