You might have a look here for a few thoughts that might help...
https://www.susans.org/forums/index.php/topic,185020.msg1645272.html#msg1645272And 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 male twin... with still the same sense of humour etc...
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
Well its up to you what you say since you know them best...
you might sit them down and talk to them... over a cup of tea... and really talk about your feelings, what moves you... staying in a relaxed tone, not making reproaches but stating your needs... just talking and asking... imo the NHS brochure might help... saying its biological, so its nobodys fault, not yours or somebodys upbringing, and its most likely not a whim or a phase... talking sensibly but saying what you need... and maybe they need some time to really understand...
they might also talk about their fears... what the neighbors might say... (there are many trans people now so more and more people get used to it slowly...)
that she had some fixed ideas about your future... well you are essentially the same person, like a male twin, with the same sense of humour... kids are possible via adoption, or possibly via stopping of hrt for some time... etc...
well as said its up to you what you say...
if this is too uncomfortable for you you might think about corresponding via email, or text... where you have time to think about it... and yet write about what is important to you... and hopefully they, too...
just remember if its written it might be shown to other persons... so I'd not get aggressive or make personal reproaches...
hugs