Hello and welcome *hugs*
Well its up to you what you say since you know them best...
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, and there are many feeling this way. Its nobodys fault, neither theirs nor that of their upbringing.
And it explains some of the feelings transgender people have.
You additionally might give an explanation of a twin... you will be essentially the same person... like your female twin... with still the same sense of humour etc...
And you might look up the genderbread person.
It shows that gender identity, gender expression, etc are different things.
Its possible to click on the picture to see it larger.
And, well, concerning talking in general it might help remain calm and relaxed, stating facts and needs, without getting upset or angry.
hugs