*hugs* well done

you could have a look here for a few thoughts:
https://www.susans.org/forums/index.php/topic,184920.msg1644403.html#msg1644403Its for a ftm person, so a few things are the other way around... like clothing for women...
Well its up to you what you say since you know them best...
talking in a relaxed manner might help... remaining calm and stating facts and a few needs...
I like the biological explanation... it means its not a whim, and its nobodys fault... not the trans persons nor the parents upbringing etc...
And the twin explanation... you will be basically the same person, with the same sense of humour, like a female twin...
some people come out in a letter... some use a short letter and talk to them later... and show some materials like some vids later...
but, well, a talk has the advantage of being more flexible...
some people drop some hints ...
I'd say take the time you need... and talk this through with your therapist...
You might play around a bit with hair and clothing styles... unisex or womens trousers, womens sweaters... etc...
you might grow your hair out... get some nourishing products for your face skin... some people prefer jojoba or coconut oil based ones for example (unless allergic)... think about sone light makeup, like a lipstick in unobtrusive colours... all possibly plant based and organic, not mineral oil based... there are video tutorials on makeup...
I'd say just take your time and do things you like, and see how it makes you feel

hugs