Problems rarely arise when a daughter-in-law joins the family. Problems arise when the family can’t agree on what role she should play. 
A daughter-in-law can be a blessing, or she can be a challenge
There’s one tendency within family dynamics that’s rarely questioned. When friction arises, the daughter-in-law’s name is often the first to be mentioned[1]. Too ambitious. Too meddlesome. Or, conversely, seen as too passive. Accusations come quickly, as if the problem always lies with the person who came later. Is this conflict really about the son-in-law, or about a family system that was never ready to accept a new member?