Is it normal for my boyfriend to look at his sisters butt?

Leave him! You can't fix him, and they don't sound normal to me at all. You couldn't pay most siblings to live together, and spend so much time together.

The cold hard nasty truth is.... We all got our very own dirty lil secrets ;)
And your bf would never admit to his own gf that he likes staring at his sister's big fat booty!!! But u seen it with ur own eyes, I seen it ALOT as well so have alot of people.
Some gurls have a ass so fuckin fat round thick and is so mf big it jus sticks out n shakes bounces when she walks and we all know that every guy she passes locks eyes on that booty and can't stop looking, we all know plenty of guys will get hard asf and horny if that ass is too fukin big n looks way too yummy! Especially if she's in skin tight jet black spandex that jus squeeze that booty and shape to it so perfectly <3 then obviously alot of dudes gonna fantasize bout it and are jus gonna want to get that booty in bed and have it all to themselves so they can do anything they want to it!!
Yess them gurls like that ;)
It's a human fact that all men will look, stare and want.
We all seen dad's stare at daughters and bros stare at mom's. ALOT of us seen dudes stare at there mom!! Many of us may have jus messed around n experimented with cuzins when we were Young! Fuk I did! After listening to our mom's get fucked dozens of times and finally watching it happen alot I found out how to watch cinemax at nite with all them movies that are jus nasty asf with sex! I started showing my lil cuzin kay all the movies and we would watch em all nite and watch her mom get fucked by different guys! I finally went in my aunts room when she was sleep after getting fucked n she was drunk and I seen her naked asf and I fingered her and she moaned! We ran out and I was so mf horny! I jus wanted to fuck!!! I started touching up my cuz and turned on porn and we started doing what they did but with clothes on!! We did this daily! Finally one day my uncle took my aunty his first cuzin and he boned her down hard asf! I couldn't believe it! Next day me and kay were alone and she was horny asf so I got a neighbor boy told him to "bun" her and he tried for long time finally I told him I'ma show him how it's done and I took her pulled pants down and all I knew was that I was gonna fuck! And I shoved it inside her and I rocked her fukin world! I was all the way deep in my own cuzin! But idgaf! I fucked her all my hardest! It was our lil secret as kids, but ayee... On her 16th bday I was 20 and she was so slutty asf! Ik she was getting the d*** tonight by anyone and dayum... It was yrs later, I done did more naughty ass shit but ayee, our momscaught us started talking shit but all i knew was I was gonna fuck tonight! And I took her in her room and our mom's were out there drinking, they couldn't stop us and bruh... They heard me bun tf outta my own cuzin! My mom seen me take her own sister's daughter in dat room and I fucked like a pornstar! Her mom my aunty is the biggest slut I ever knew but I wanted it bad asf n finally got it when she was drunk! Yes I fucked my mom's sister and my own full blood 1st relation cuzin! Cuz they were lil sluts! But I seen my cuzin Jared take his own lil fine ass sister and watched him fuck her as hard as he cud!! I fucked every single one of my Aunty's who were fine asf!
Moral of story is my sister had the best ass ever! Forever and I always wanted it! I done felt it up, grabbed it, smacked and squeezed it! I dry humped tf outta it one nite finally one time I felt the inside of her pussy ;) finally I wanted that ass so fukin bad!! I was horny asf n drunk and she was drunk asf passed out ass up looking good asf! And yeah I pulled them pants down and I went in ;) OMG! I was inside my own sister!!! But fuck yeah I fucked her as hard as I could ;)

I wish I had a hot sister.

Well... IT'S NORMAL for him to stare at her butt, if she has a nice one, then it's natural for a guy to notice a nice butt and tits (yes, even brothers and sisters). My brothers are hot, and I notice them. I also catch them starring at me (ever since my body started to 'develop').

Sit down and talk to them both, together, maybe the issue will surface and get resolved. Perhaps suggest to your BF that since he likes her butt, then maybe she could join during sex - he'll either take the suggestion seriously, or stop starring at her. If you suggest to her about joining you and her brother during sex, then she'll either like you very much, or continue not liking you. Either way, you'll know what the issues are. All seriousness, talking to them will help.

Maybe your just reading to much into it it's not uncommon for a girl whose close to her brother to feel jealous of a girlfriend. And it's possible that he wasn't looking at her ass

Don't most girls usually where short short when they r home.my ex used to always where my boxers at home it was kinda cute

If she has said those words to you obviously something was going on maybe using you as a cover up???? If you where mine I would make sure I tell YOU how nice your butt is

Maybe they deserve each other?