Robert is married to Linda, and they have two sons, Marcus and Alexander. Robert has one sibling, a sister named Sarah, who has two children of her own: a daughter named Rebecca and a son named Kyle. Linda is an only child. Explain the relationships between Sarah's children and Linda, and between Marcus and Rebecca. Provide a very brief response.
Rebecca and Kyle are the children of Sarah, who is Robert's sister. This means Rebecca and Kyle are Robert's niece and nephew. Since Robert is married to Linda, his niece and nephew are also Linda's niece and nephew by marriage, not by blood. Therefore, Rebecca and Kyle are Linda's niece and nephew through her marriage to Robert.

Marcus is the son of Robert, making him Sarah's nephew because Sarah is Robert's sister. Rebecca is Sarah's daughter, making her Robert's niece. Marcus and Rebecca, being the children of siblings (Robert and Sarah), are first cousins to each other.