What is the ruling on sisters kissing their brother, and their presence in his lap or sitting on his knee, and what is the ruling on a maternal uncle kissing his sister's daughters on their lips when they are seven years old and above?

It is permissible for a brother to kiss his sister on the cheek, provided it is not done with lust. It is also permissible to embrace her if there is no direct touching of 'awrahs (private parts) and there is no fear of temptation. However, it is not permissible for a sister to sit on her brother's lap, because the touching of 'awrahs is forbidden, even with a barrier. As for kissing children, there is no harm in it if it is motivated by affection and mercy.