That's very sweet of you to think of your family member struggling with infertility. But like pp have said, you can't put your own life on hold for hers. I'm sure if you decide to try again and get pregnant before her you'll be the sort of person who is very considerate with such a delicate situation. I'm sure she'll be jealous and possibly feel quite badly about it, but you have to do what feels best for your family.
I didn't really think about my fear of having a third boy while we were TTC, but around 8 weeks or so it really started hitting me, and I too would get into a total panic just thinking of the ultrasound. I would spend hours imaginging the two possible outcomes and how I would react and how I would feel afterwards. I did Acu-Gen knowing they might be wrong, but couldn't help myself - I wanted some kind of answer one way or the other so I could prepare myself, in some ways I was really hoping they'd say "boy" just so I could convince myself of it even though deep down I knew they could be wrong. We made a whole trip to have an early u/s done at 15 weeks, and the night before in our hotel room I was lying awake for hours thinking about it, thinking I might even have a panic attack lol.
After all that worrying, it was a girl after all (which I am still having a hard time actually believing lol - can't wait for my regular u/s in 2 weeks so we can make sure!) I can only hope everyone gets the outcome they're hoping for, but of course that's not going to happen. But you never know - you might be worrying for nothing, and there might be a little girl just waiting to join your family whenever you're ready to try. Or another little boy you'll grow to love and wonder how you ever lived without. But you'll never know if you don't try! =) Lots of luck with whatever you decide!