I thought Lin was mad because her sister's life is so seemingly effortlessly perfect and everything has worked out for her (don't you just hate those people?). It seemed too petty a reason for her to have the full-on hate going though. Not being able to have kids is a reason that fits really well, especially considering how she treated Opal. ...I still don't think that's a very good reason to hate her family (in fact, I'd think it should be the opposite - can't have kids, so spend a lot of time with her sister's), but I guess we'll see if there's something else going on altogether.
I just get such a vibe from Lin's sister, like she's going to somehow ruin everything she's worked for her whole life in a huge display of freakout. Possibly while trying to overthrow the Earth Queen, as people have been speculating. And then Toph will come and sort them all out and it'll be all happy and great again and yay Toph (my favorite TLA character)!
Oh, and I really wanted to see Lin vs the Dai Li, it'd be like Toph vs Dai Li (round 2)... but alas.