Big whoop. How does this help 'prove' how Laci & son were killed & by whom? Nothing probative in a father not liking a son-in-law, or his love for fishing where others fear to tread.
By itself, it doesn't mean much. But as one attorney put it, think of it as another brick that, with all the other bricks, builds a structure. Circumstantial evidence can be just as damning as, say, a videotape of the actual murder, if put together skillfully.