You know you don't get along all too well with your brother when you can fill 3 whole blockbuster movies about your quarrels. True, we all fight with our siblings, but a little more about breaking each other's stuff or ratting each other out to your parents than about killing each other's girlfriends or destroying the world. But then again, Loki is adopted, so there's that whole baggage.
The Bluth siblings don't really hate each other as much as they just... don't care. Every Bluth is busy thinking about themselves to give a crap about what their siblings are feeling. This family is all about sibling rivalry. Let's not forget they got famous for an actual video-tape series called "boy fights" (And baby Buster).
Oh, brotherly love. Wait, no. That's too much love. Stop it.
The Lannisters are known to take their feelings to the extreme. If they love each other, they'll have sex. If they hate each other, they'll try to get one another murdered. Not too big on the subtlet with this family.
Many siblings fight when they have to share a room growing up. So what about sharing the same body? Bette and Dot don't get too much time apart, so tension could run high. Add to it that dot is frequently open about her dream to slice Bette's head clean off and live a normal life, and you got a not-too-ideal living situation.
Speaking of siblings who wouldn't mind killing each other, Dennis and Dee never really showed any affection towards each other in 10 seasons of the show. Dennis and Dee have set each other on fire, got each other addicted to heroin and tricked one another into completely destroying their relationships. Even when they do try to bond, it's by doing not such healthy things like digging up their dead mother's corpse.
These two went from being best friends, to almost killing each other, to not speaking, to again almost killing each other and then to kissing in order to save their lives. So, a lot of family history there. Basically, they get along okay now, but they deserve a room on this list, if only for the things we saw them doing in Tumblr fanart.
A lot has been said about this New York brother and sister pair. They share way too much, hang out way too often, and sometimes fully forget they're even related. This is so apparent that even Joey found himself asking Ross if he would sleep with Monica, not understanding why that's wrong. Not to mention the fact that on a later season these two found out they kissed at a frat party.
At least the rest of the people on this least are completely fictional. What's your excuse???