How can one deal with a ten-year-old sister who does not always adhere to being separated from playing with her maternal uncle's sons and others, based on the recommendation to separate girls from boys in education and play at the age of seven?

The legal Islamic rulings must be observed when distinguished boys and girls play together. You have done well by guiding the ten-year-old girl to avoid playing with boys, as she is at an age close to puberty. You should continue to advise and guide her with a good approach. If she does not stop playing with them, her guardian must compel her to stay away from them as a preventative measure (sadd al-dhari'ah), because children are trusts with their parents, and Allah has entrusted them with these children so they may teach them, discipline them, and protect them from corruption, in fulfillment of His saying, the Almighty: "O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones."