Distant Cousins (1993)

Everything's going great for ad man William Katt and his luscious lady Mel Harris; they've got a low-key life and a lovable tyke (Brian Bonsall), but things are about to get dysfunctional in Distant Cousins (1993). That's because Will's long-lost cuz David Keith and his fine fiancée Marg Helgenberger have come to stay. At first they seem okay, but it's not long before things start to go horribly astray. Dave and Marg are one seriously unstable couple, and they're deliriously jealous of Mel and Will's and idyllic existence. Should they see a counselor? Try to get their acts together? Split up and find other lovers? Nah, that all takes too much time. Effort too! Why don't they go ahead and just steal the life of cuz and his wife outright? Starting with pilfering Brian's affections, that's just what these psychos do, until finally the chicanery turns deadly.