Hi Rosey,
my sister is trans. Came out as trans at about 25. A difficult decade began for our parents - my sister lived at home and worked at the same company as they did. At home she would go as 'she' and her new name, but at work remained 'he' and old name - was not 'out' at work. So my parents were constantly switching around how they referred to her! DSis was having counselling and it wasn't going well - her counsellor was a dick an old fashioned conservative who thought that if she didn't have long hair and wear dresses and date men, then clearly she couldn't possibly be female. After much too long a time she moved to a new counsellor, came out in public, and started passing as female. At age 40, so 15 years after first coming out as trans, she had surgery and was able to get documents in her new name and gender.
But those hard transition years were nothing against the preceding decade, which was characterised by suicide attempts, anorexia, life-threatening self-harming, drug using, depression, manic phases, a number of admissions to mental health hospitals, flunked education, flunked jobs, ...
You're understandably thinking about the impact of your child's transitioning on your other children. And I won't say it's easy. And yet - if your child really is trans - it is probably easier than the alternative.
Using different pronouns and a new name is quite a challenge, and now that I know quite a number of trans and genderqueer people, I can say I find it harder when I have known a person for a long time under a different pronoun/name, but also when a person has a physical appearance that is very clearly not in accordance with what we expect from people of their gender.
But allow yourself adjustment time, learning time. Don't be too hard on yourself if you slip up. I know it meant a huge, massive deal to her that I consistently called her 'she', and so I gave my very best - but still I slipped up sometimes. As time passed it did get easier, because I started thinking of her as my sister (rather than my brother-turned-sister).
Regarding dating and such, my sister has over the years had very good relationships with all sorts of people. Some genderqueer, some trans, some cis; some straight, some gay, some undecided - but all of them extraordinarily nice people who judge a person by who they are, rather than by what they have between their legs. I think that's an excellent quality in a person. Her trans-ness has meant that she only got to date nice people.... so to say.
She is now happily married by the way.
Regarding young children, my own children have only ever known my sister as their auntie, but my eldest does notice that auntie is a bit different. And has met a number of other people who don't fit nicely into the gender binary. They take it in their strides. We have been careful from early on how we talk about things, e.g. when DS was adamant that he would grow up and be a mummy, we had all sorts of conversations. For example, that most boys grow up to be men, and most girls grow up to be women. Or that nobody can tell them who they can and can't be. We explained that he would never be able to have a baby grow in his tummy, as he didn't have the right kind of tummy, but that being a mummy is so much more than growing a baby in your tummy. Also lots of talking about e.g. 'colours are for everyone' and just fighting against the notion that anyone should be able to limit your options due to you being a boy or being a girl.
As to your older children, my own experience as a sibling of a trans person was that the thing I found most difficult was that when I grew up, I thought I had a brother. But as it turned out, my sister always did feel she was a girl, and struggled with 'being a boy'. So in effect I had a sister in disguise, but didn't know that. So I had experiences growing up such as my 'brother' being allowed things such as crazy hitch-hiking trips and wild camping by himself, which I wasn't allowed due to it 'being to dangerous'. My 'brother' was extremely bullied at school due to desiring to have long hair, I was of course never bullied for having long hair. These are things I now find very hard to talk about - it doesn't make sense without knowing all the back story. So people who just know that I have a sister, how can I talk to them about my sibling being bullied for having long hair? I experienced growing up with a brother but it turns out I didn't, actually. That's a bit weird. I grew up with a sibling who was seen and treated as a boy (positively and negatively) but who all the while didn't feel they were a boy.
Perhaps the difficult thing is that it kind of feels like my memories, my experiences feel invalidated. Things weren't what I thought they were. Maybe that's a bit like discovering at age 18 that you've been adopted. It IS hard. So I wouldn't be surprised if your other older children struggle with this. They may be angry, or choose to have some distance for a while. I think it is important to accept THEIR feelings as valid, too.
But again, that has become easier over time, and my own struggle with this is nothing compared to my sister's struggles.
As to comparing it with a bereavement: Well I think it is a 'loss' albeit not a death. My younger sister did indeed die - so my parents were all too familiar with 'losing a child'. And I knew what it can mean to 'lose a sibling' (although I was still little when it happened). It's different when your child/your sibling transitions. What you lose is an imagined future, not the person. I think it may be a bit similar to the feeling of loss when you are told that you are infertile. People are told to give themselves time to grieve the future they won't have, e.g. before making big decisions for example adoption. Unfortunately you don't get the luxury of time - your child is there, right now, and needs you. Needs you to embrace the new future. Because of course, whilst losing the old future, you gain a new future - a future which in my case entailed having a sister!
However it takes a while to wrap your head around that new future. And the fact that you have acquired a new future doesn't change the fact that you have lost the previous imagined future. So given that you don't have 'time to grieve before you move on', you will have probably have to do both in parallel. But don't forget that it is a hard thing you are doing, grieving and embracing a new thing all at the same time, and being there for your other children with their various needs too. So please, go easy on yourself!
I'm sorry if this is all a bit too much right now. You're only three days in! It's been 15 years for me so a lot easier to see from a bit of a distance. Thinking of you - you sound like a great mum.