My youngest sister, 28, is in a similar situation. I have wondered myself about her. I wont ask because she will say no, and I don't think it will help me to stop wondering. Unless I see her with a man of course.
It sounds like you have a good relationship with your JW sister. You should be able to talk about your concerns for her happiness and reassure her you will always be there to talk to. Talk about her interests in marraige and children and not directly about homosexuality.
Maybe she has a desire for a male companion, to be married and have children. In the small group of JWs she is familiar with there may be no offering. I was there once when I wanted a female companion so I know how disappointing that can be.
Some part's of people's lives are private. Unless she is in physical danger, respect that privacy.