I've just had the weirdest conversation with my SIL. DH has a brother , Tom, who has had children with two women, neither of whom he has married. SIL is married to another of DH's brothers. We have just had SIL over for the exchange of Christmas Presents. MIL was here as well. MIL laid into SIL for not buying presents for Tom's children. SIL said that they weren't her relations as Tom had never married their mother and she was only related to the family through marriage. The whole situation got a bit heated. SIL says that they are nothing to do with her. Admittedly we never see the children, MIL is the only one who has contact and I always feel a bit odd buying them stuff, but I always thought that I was their Aunt, but perhaps I was wrong.