7. Mr. Niu Deheng, his sister, his son, and his daughter are all tennis players. Situation: (1) The twin of the best player is of a different gender from the worst player; (2) The best player and the worst player are the same age. Then, among these four people, the best player is $\qquad$
Mr. Niu Deheng's daughter.
From the problem, we know that the best player and the twin sibling of the best player are the same age: from (2), the best player and the worst player are the same age: from (1), the twin sibling of the best player and the worst player are not the same person. Therefore, among the four people, three people have the same age. Since Mr. Niu Deheng's age is definitely greater than that of his son and daughter, the three people with the same age must be Mr. Niu Deheng's son, daughter, and sister. Therefore, Mr. Niu Deheng's son and daughter must be the twins mentioned in (1). Thus, Mr. Niu Deheng's son or daughter is the best player, and Mr. Niu Deheng's sister is the worst player. From (1), the twin sibling of the best player must be Mr. Niu Deheng's son, and the best player is undoubtedly Mr. Niu Deheng's daughter.