This is what I found search family attribution rules.

From IRS " Attributions of Ownership Rules"

A member of the family includes any spouse, ancestors, children, grandchildren, great grand­children, and spouses of children, grandchild­ren, and great grandchildren.  A brother or sister of an individual is not a member of the family for this purpose.  A legally adopted child of an indi­vidual will be treated as a child by blood.