Hi, I'm not a mum, I'm am LGBTQ+ teen and I hope that I can help in someway.
Please, please, please call your child by the correct pronouns, do not deliberately call your ds dd. If they say that their pronouns are 'he/him' not 'she/her' you must refer to them using those pronouns. If you accidentally use the wrong pronouns it's forgivable, as long as you learn from your mistakes and correct yourself as soon as possible.
If you believe that it's too soon for name changes, i suggest making a gender neutral nickname. Try not to use ones like 'darling' or 'sweet-heart' etc as these sort of ones have inherently female connotations which can make someone feel uncomfortable and I assume that it's in your best interests to make your ds as comfortable as possible. Also, if they do start to mention about changing their name, you could always try and get them to lean towards unisex names like 'Sam' or 'Alex' etc, this might make it easier for you to adjust.
You might not feel quite comfortable with this yet, but you absolutely cannot show that to your child, unfortunately, you've got to do that by yourself, or with someone else who feels the same as yo, but it's a definite no to letting your child know how you really feel, because coming from experience, it can lead to bad bad things (self-harm, increased suicidal thoughts, increased likelihood of an eating disorder, the list goes on...)
Don't get me wrong, I'd probably be a little shocked if my child told me they were transgender and I understand that it would take some getting used to, you've watched them grow up and you've thought about what their future might look like, and suddenly you're hit by something you didn't expect... If I were in your position, I'd talk to them about it, and although it might not be what they want, I'd wait a little longer for them to think about their transition. I'm glad that my parents did anyway, I used to tell them that I wanted to be a boy (from about 5-10/11yrs) but now that I'm just a little older, I've realized that i'm just gay.
Let them wear the clothes that makes them feel comfortable, it will make it easier for them.
You don't have to tell your 12yr old that their sibling is transgender, but explain the concept of it to them, explain some other LGBTQ+ stuff to them too, it'll be beneficial to them an hopefully more people that they'll meet in the future (if you'd like any knowledge on the lgbtq+ community i'd be happy to help!)