TLDR at bottom
Background: I’ve known my ex (J) since 2013.
I currently pay rent and live with J’s mum, J’s step dad and J. J and I are still best friends post breakup.
J’s family has had this dog for the last 8 years, and for the last 5 years it has always shit and piss inside the house and none of them bothered to retrain her.

J’s little sister (R) moved out Easter 2018. Occasionally she comes back to stay for a few days.

R shows up out of nowhere unannounced, and sends a Facebook message to myself and J saying:
“Idk if you guys are here but I'm at home in town tonight and until Fri or Sat”. 

Then starts the problems. 
Spare bed upstairs isn’t made, she complains to us that it isn’t made. R is 21 in less than 2 months. She lived in this house for 4 years before moving out. J tells her to make the bed.

20 minutes later R says:
“Also do you guys not know how to pick up dog shit and air out a house. That shit i just picked up looks like its been sitting there for days.”

J and I had spent 2 hours cleaning the entire house, and there was no dog shit there at 8:30pm when we went to bed.
I have always in the past just let her walk over me. I decided enough was enough. 

Me: “Jfc R we’ve been in bed since 8:30 which is more than enough time for her to shit
The house has been open every day, if you have a problem remember she’s YOUR family dog”

R replies:
“The shit was white and crusty it did not just come out of her ass, hence why I made the comment 
No need to go off on a tangent and be rude.
You literally haven’t even seen me to say hi and you’re already having a go at me when I’m literally not even mad?
HOME SWEET HOME HEY!
Remember who’s house you’re actually in”

My reply:
“Well it wasn’t up there when we went to bed.
You were the one being rude R, “don’t you know how to pick up dog shit and air out a house” is pretty fucking rude, and I’m not putting up with that shit.
Sorry for not getting out of bed at 10pm to come and say hello? 
Temember that it’s you MOTHERS house, NOT yours, and I pay RENT to live here, which means I’m entitled to be here”

R replies:
“It’s called sarcasm darl 
Stop trying to fucking argue and prove a stupid point haha
This is literally so dumb, come talk to my face if you have further irrelevant &amp; petty shit to say”

My reply:
“Hmmmmmmm or you could’ve just dealt with it and been an adult about it
I don’t need to prove anything, you’ve done that yourself.
“Remember who’s house you’re actually in” ?
Think you’re the one being petty R, wake up to yourself.”

AITA?

TLDR; my ex’s little sister (21) came to her parents house where I currently live and pay rent and ex lives here too, proceeds to complain about dog shit on the floor which wasn’t there when we went to bed, I call her out that it’s her family dog then call her out for being rude and petty after she tells me “remember who’s house you’re actually in”