As discussed here, Reish Lakish is of the opinion (in Yevamos 10b) that one may not marry his sister-in-law with whom he did chalitzah. He derives it from the passuk of "lo yibaneh" - literally, he shall not build. Because he did Chalitzah, he may not marry her again.
Why is this not a normal arayos case? One may not marry his sister-in-law. Chiyuv yibbum suspends that issur. Once he's rejected that opportunity, she should return to the parshah of arayos. That would mean that A) the passuk of lo yibaneh is unnecessary, as he is already prohibited from marrying her, and B) it would be an issur kareis, which is contrary to the opinion of both R' Yochanan and Reish Lakish in the aforementioned Gemara in Yevamos. Why is this not so?
(Note that yibbum is one of the classic cases of asi aseh v'dachi lo sa'aseh - see Rashi to Shemos 20:8, as discussed here.)