This definetly seems like a double standard to me.It is sad that in society in general its ok for a guy to date a younger girl,but not visa versa.Sounds like the religion warped his views on sex,relationships,and trusting anyone.He should not tell your sister she shouldn't date.Why can't he understand that all humans,male or female have needs of every kind.It isn't all about sex all the time for everyone,although that is fun.He souns jaded toward men who aren't JW's.
My father often said to me"aren't you glad your sisters aren't whoring around?'I say,well they should do whatever THEY want to do.Having sex with a boyfriend isn't being a whore,and even if they chose to sleep with multiple people,thats thier decision.
He is a hypocrite with a wrong view of the world.IMHO
STP