You sound like a lovely person, please don't give up on her! It does sound like she's had a rough time and possibly doesn't know how to deal with you being kind to her, but perservere and one day she will. Don't confront her, just carry on being your lovely self. She will be grieving more than just her mum (which is bad enough for her) but possibly the fact that she didn't have such a loving and close relationship with her mother as her husband has with you. Keep making a fuss of her, buy her little gifts, make her drinks and treat her with kindness. I don't think she means to be rude, she maybe hasn't been brought up in the same way as your son, but they love each other and that's a good start. Well done for being so patient, I'm sure one day you will be great friends.