Trump was sent to military school at the age of 13, which would have been in 1959. If he sexually assaulted anyone at that age, I think it's vanishingly unlikely to have been a family member, due to lack of opportunity.
Both of his sisters are older than him. The oldest, Maryanne, was 9 years old when he was born; Elizabeth was about 4. That means they were 22 and 17 when their 13 year old brother was sent away. There wasn't much extended family to speak of, due to their mother being a recent immigrant to the U.S. and dad being the son of immigrant parents. His only other close female relative is niece Mary, who wasn't born until 1965.
On the other hand, he was expelled from the private school he had been attending due to bad behavior at school, and we don't know what all that included. So feel free to speculate there! 
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