What is the name of movie that Jane Seymour in where she caring for her nephew and then marries his father?

The movie you're thinking of is "Wedding Crashers" (2005). While Jane Seymour doesn't play the character who cares for the nephew, she does play the role of "Kathleen", the mother of the nephew (played by Owen Wilson) and the love interest of the father (played by Vince Vaughn).

The movie features a hilarious plot where two professional wedding crashers find themselves falling in love with the bride and groom's families.