Cousins (1989)

Synopsis
In this romantic comedy, two people brought together by marriage are brought even closer by their mates. Maria Hardy (Isabella Rossellini) and Larry Konzinski (Ted Danson) first meet at a wedding, where Maria's mother and Larry's uncle are tying the knot. However, the new cousins also have something else in common: Maria's husband Tom (William L. Petersen) is having an affair with Larry's wife, Tish (Sean Young). Maria and Larry get to talking at the wedding reception after their spouses go missing for a while, and they develop a rapport. A friendship grows between them, and they start seeing each other on a regular basis. When Maria confronts Tom about his infidelity, he responds by asking her if she's sleeping with Larry. As Maria and Larry become aware of what's happening between their not-so-better halves, they decide to get revenge by pretending to have an affair as well. However, the longer they pretend to be in love, the more they realize that they aren't pretending after all. Cousins was based on the popular French film Cousin Cousine. ~ Mark Deming, All Movie Guide