You can, but that would be wrong in so many ways.
His father
Chris Gallagher.
He had a good relationship with his sister
His name is Brian Teefy
You sister-in-law is your son's aunt. the relationship consider age diff.if she is lower age what is his relationship.
My relationship with a friend's sister is friendly and respectful.
I am a third-party voice and do not have a relationship with your friend's sister.
Your husband's sister is your sister in-law.
she is his younger sister
Your Mother's Sister Is Your Aunt.
Your sister's daughter is your niece.
Your sister and your wife's sister are not related to each other.