First of all the writer is the wife of the young woman's father, she is not her stepmother. Just because the father married this woman, his daughter is not obligated to accept her and her children as family.

There were several clues in the letter making it obvious the young woman is not interested in being "included" as a part of this woman's family.

The new wife needs to enjoy her relationship with her husband, and allow him to see his daughter and grandchildren without her tagging along.