The rights of the step-parent regarding the step-child are something every family has to make their mind up about individually. However, by what you describe, I feel your girlfriend oversteps the boundaries of parenting by far, which may lead to the severe consequences you have related. I would expect it to be everyone's right to step in and get help (even from "outside", if necessary). Stick to your guts to decide when "it's enough".

The high suicide rate is high on all transgenders, not just "teen transgenders", before and after the "transition". In fact it is practically identical to the rate of suicide among mentally ill people... if I were you I'd suggest your child to go to a therapist to discuss this decision, not to try to change his/her mind but to be sure that it's a mature decision not due to some other psycological issue.

Another comment that is not quite an answer: legal guardianship is probably the wrong way to go, as mentioned in @anongoodnurse's answer. Counselling would be my first idea.

@Bakuriu - this! Transitioning is a huge deal and since "trans trenders" are a thing, you need to make sure she actually needs the help and isn't just gender-role nonconforming. Otherwise she risks going through all the stress of transitioning just to realise that it hasn't helped fix her problems.

Did that all came to you as a surprise? Did you see signs? Before doing anything, you need to make sure that everything is true and not just an act of rebellion, which can happen in families with problems.

@the_lotus I was not surprised at all tbh. I left out the signs I'd noticed for the sake of brevity. But she would cry and sink into despair over "little things" like my GF forcing her to get haircuts and wear overtly masculine clothing, not letting her wear any jewelry (even the types of watches or chains common for men - which I wear!). I don't deny it can be a rebellion (common even among families with no "problems") or a phase, although I have no reason to think that. But that's why Emily will be going to therapy anyway.

@Charlie these look like normal sign from an overly controlling mother.

@the_lotus No, I never heard of a cisgender boy bawling his eyes out over getting a haircut or not being allowed to wear a watch. As I said, I don't deny it can be a rebellion or a phase. That's up to her future therapist though, not y'all armchair therapists

It sounds like you've been doing this already, but I recommend reaching out to the local PFLAG (or trans equivalent) for advice. Unaccepting parents are something that they have dealt with many times over and I am certain they can offer some good advice.

I agree with @the_lotus. This sounds like Why am I not allowed to wear/look like this... Hey, actually, these things are 'girly' and not 'boy-y' and that means I'm a 'girl' rather than a 'boy', that's what's wrong. In any case, the mother is to blame for this situation.

I am somewhat confused by what you said and found something unclear. Does Emily have XY chromosomes or XX chromosomes? Your use of bother gender terms left that question in my mind.

@NoctisSkytower Why does that matter for the question? It is clear that 1) Emily has been assigned male and has biologically male hormones and 2) Emily identifies as a trans woman.

It might not matter for answering the question, but I was still curious what the person's DNA had to say.

@Bakuriu Please refer to us as transgender people, not "transgenders" -- like "gay people" vs "gays" or "black people" vs "blacks".

@Pyritie What would it matter if she is or isn't "just gender nonconforming"?

@cat Why? English is not my native language and in my mind they sound exactly the same, as do other things like Italians vs Italian people.

@Fiksdal The question says Emily's father passed away before she was a year old, so her mother is her only surviving parent.

@Basic Thanks, missed it.

Do I misunderstand something -- how can OP be a step-parent if not married to the mother? OP's standing sounds (legally) precarious, should the girlfriend really dig in. Definitely need some outside help, even legal help, for Emily.

@Jeff Y Right, I'm not married to Emily's mother, but we still refer to each other as step-parent/step-child. It's a social term, not a legal one. I didn't know it was illegal or legally precarious to date a single parent. If so, that's alarming. I'll consult with an attorney...

"my girlfriend feels I have no say" - Emily apparently feels you do. Don't give up on her, or her mom. Everyone in this situation needs some love here.

@Charlie I meant "legally precarious" only if girlfriend desires to boot you out over the situation. I.e. she hold all the legal cards here. If you were married, it would be different. (I didn't mean there's any legal risk in dating a single parent by itself.)