Hello and welcome *hugs*
well being in the UK you could contact the mermaids, its a group specifically for transgender youth:
mermaidsuk dot org dot uk
"We offer information, support, friendship and shared experiences."
They can be contacted via mail or phone...
you could ask there for more information and support.
And you might look up a brochure called "doh-transgender-experiences.pdf" . Only thing I would disagree with is page 7, where they state stress, instead many experience relief.
It states that being trans has biological connections, to do with development before birth which influences the sense of self.
So its not a light hearted decision but how they feel, and there are many feeling this way. Its nobodys fault, neither theirs nor that of their upbringing.
It explains some of the feelings transgender people have.
I personally also like the twin explanation... parts of you will stay the same, like a male/female twin... with still the same sense of humour...
Well since you know them best its up to you what you say at home... you might think about showing the brochure, its for the british national health service, a reputable source...
talking in a relaxed manner might help... remaining calm and stating facts and a few needs...
some people come out in a letter, and show for example some vids later...
relatives might also talk about their fears...
for example what the neighbors might say... (there are many trans people now so more and more people get used to it slowly...)
that they had some fixed ideas about your future... well its all possible regardless...
well you might try to play a bit with hair and clothing styles, with for example some trousers of your preferred gender, and see how that makes you feel...
but you might ask for more at the mermaids...
hugs