Sometimes it take years

I have an adopted son, and my sister has an adopted son. Of course, we sisters were raised with the same style of parenting, so I can't believe that we are raising our children that much differently. Our son is accepting of loving and for the most part, was his whole life. My sister's son is only now (age 22) starting to accept her friendship and love. Do you think this is simply because of individual personalities? They were both adopted as infants. Both multi-racial.