It could prompt them to look into that therapist's specialties to see why you want that particular one, since you're 18 I was operating on the assumption you'd pick one and go alone, every family is different I guess. In that case you could try talking to the same therapist your brother sees, even if they don't specialize in transgender issues they may have some experience there anyway, obviously take them with a grain of salt, if they immediately try to steer you away from trans something is up, since they can't really make that judgement so easily. That said it's really important that you're comfortable enough with the therapist to talk about these kinds of deeply personal issues, and it sounds like you're not completely comfortable with that one... Most of the value of a therapist is what you say to them, not the other way around.
As for the reason, pick a half truth, maybe you're bullied at school, have anxiety, depression, etc. Being trans almost always comes with another issue or two.
There was a whole paragraph here about how women come in all shapes and sizes, but that point was pretty well taken care of by stone.
Just know that hormones will change your appearance a lot if you're going to use them, you can't really tell how you'll look in two years based on now...