Is it normal my friend won't shut up about her sister-in-law?

I love my friend but she won't shut the hell up about her sister-in-law. So she's not her sister-in-law yet but she's engaged to be married so she will be her in-law.

Anyway, every time I hang out with her, all she ever talks about lately is how her sister in law dresses, acts, etc. She'll say "My sister in law just got a nose piercing and it looks ridiculous", "She told me that her sister in law wrote " I want to f*ck you with my big black c*ck" on facebook" and always mentions how she dresses slutty, she wears this and that and this and that, etc.

My friend is 24, has been with her man for 4 years now and plans to marry in 2017. Her sister in law is an immature 19 year old. I get that, you know. I get how it can be annoying because she's wild and not sensible like she or I was at 19 but I don't know what the hell to do or if this is normal. Is it really normal to vent all the time to me? I can only take so much. I don't even know what to tell her. I guess I can't relate because I've never had a serious relationship in my life yet so I don't know what it's like to not get on well with the only sister of your boyfriend but still.

Am I overreacting? Should I advise her to do something? I tell her to talk to her sister in law and she has tried to have serious talks with her but she swears a lot and I remember she told me she had said something like, "I'm a f*cking adult and I don't give a sh*t what you think. If I want to go to clubs at 3am and get laid, I'll go to clubs at 3am and get laid"...classy, I know but yeah, she's pretty much just immature so I don't think there's any other advice I can give her to say to her sis-in-law.

IIN?