...*giggle*
Anyway. *cough*
Yes. He did kill her. It's a little-known, VERY little-documented fact, but he had a grudge on Turin and so sneaked all the way over to his house to kill his baby sister. A cos he's just awesome that way.
Really, though, Lalaith's death had nothing to do with the Feanorians. For one, why in the world would Maedhros have wanted to kill a little girl? For two, Maedhros (while awesome) did not have the ability to predict the future or to read Morgoth's mind. How was he to know a) that Morgoth would send a plauge or b) that it would kill this one little mortal girl in particular? He wouldn't have known a) and he probably wouldn't have cared about b) except in the context that she was one of many. I think, in order to say that he "killed" her, he had to have actually cared/known about her death.