Hugs and welcome

You could look up the following resources for a few thoughts:
there is a website called gayteens dot about dot com where there is a quiz "am_i_trans.htm" . It might help you with a few thoughts, they state a few opinions of others, and explain a few terms. And it might help by showing you are not alone in this.
And you don't have to feel male all the time. A question would be would you prefer a more male body to be more happy ?
And there is a pflag brochure "Coming Out As Trans.pdf" with a few points on the first page... the second page is more for own children..
I personally like the twin explanation, you will be like your female twin... with still the same sense of humour etc...
And you might look up a brochure for the british NHS 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 some biological connections, which is imo socially a bit better acceptable, and it might help with self acceptance.
Well since you know them best its up to you what you say... you might even think about showing the brochure, its for the british national health service, a reputable source.
Just remember there are many who have gone through this and reached their goals eventually

.
You could play around a bit with hair and clothing style... mens or unisex trousers, sweaters one or two sizes bigger... second hand stores could be a good source. It can be really fun.
Please reach out...
You might think about talking to a counselor... or a counselor at school you trust, or at a school gsa...
and you could call at the next lgbt center near you and ask for some counseling...
there might even be support groups...
you might say at home you want counseling, for emotional purposes, and pick someone who has, amongst others, gender on their list...
and if you want to talk to someone you can call one of those for example, people are there to help and give advice (there are many others):
glnh dot org/talkline/ (this is the national lgbt youth helpline)
translifeline dot org
Sometimes its dependent on the person on the line.. just keep at it and reach out...
Well its a process... I'd say take the time you need, but keep at it...
and come here, people will support you

hugs