Like Janet I think you should tell them when you feel that it is right. I understand totally the feeling of wanting to share your truest and deepest self with your family. I also get the feeling of frustration and uncertainty. On the one hand you want to do it, but on the other you don't want to hurt them, or accidentally make things worse for yourself.
I told my family whilst I was still young enough to be in pre-school. They didn't believe me back then... (who believed what a five year old says) but because they were an understanding and loving lot they went along with it to a surprising extent and as a result I had a good childhood living sort of in between genders.
On the one had the sooner you tell the easier it will be to do, and once you have it out the way then there is nothing lurking in background as a big scry moment.
On the other hand, if you tell them before you have even seen the counselor they are probably apt to say "yes but the counseling may change your mind" so you have to be aware of that and be prepared to be gracious and not defensive in your reply. In other words a good reply might be "well it may do, but I would be very surprised if it did..." and not "No it won't!" - if you see the difference in emphasis there.
Whatever you decide I wish you good luck. I would say that from the fact that your family have hooked you up with a counselor they are good people who have your best interests at heart - so I'm pretty certain that like me and many others of the fortunate minority, you won't have to face too much in the way of hostility and rejection from them - which is great.