My 14 year old step-daughter came out to me (her step-dad) as a girl on a bike ride when my girlfriend of 3 years, her mother, wasn't there. I fully accept her and gladly refer to her by her chosen name ('Emily' for the purposes of this question) and pronouns. Emily told me she hadn't told her mother yet and she felt more comfortable telling me first. Overall, we have conflict-free, loving, pretty great relationship. She told me not to tell her mother and of course I agreed.

The bike ride was on 9/11, and Emily finally came out to her mother on Friday night (5 days ago). I don't know what exactly happened, I got home late from work and they were in a heated argument, but it ended with Emily going to her room and refusing to come out.

Later that night, my girlfriend referred to her own child as a "degenerate". She's mentioned "sending him to a camp" and disowning "him". She has made other similar comments as well as creating an unsupportive environment by misgendering Emily and referring to her by her former name at every opportunity. She's become even more strict about Emily's clothing, hair, accessories, etc. I would have understood if she were shocked or in denial, if she thought it was a phase, etc. but to be honest, she might well bully her own child into suicide at this rate.

I have tried to talk to my girlfriend, she only responds that I am "taking his side" and "promoting that lifestyle". I tried everything from telling her about the high suicide rates among trans teens, I showed her studies on how trans teens start doing so much better (in terms of mental health, in school, etc) once they start treatment ¹, and others but she is not budging. There is the added issue that Emily is not my kid, so my girlfriend feels I have no say (and to an extent, I agree).

My question is, how do I best protect Emily not only from bullies outside the home, but from the bullying she's experiencing from my girlfriend? I don't want to get between Emily and her mother. Emily's father passed away before she was a year old, so her mother is her only surviving parent.

Also, if it's relevant, Emily is the only child in the home - I don't have any children and she's my GF's only child. They moved in with me about a year ago.

Second question: My girlfriend's talk of disowning or sending Emily to a horrific "conversion" camp has me wondering if I should be taking steps to become Emily's legal guardian (if she agrees). How do I discuss this with her and what are the benefits of legal guardianship in this situation?

¹ Emily told me she wants to see a doctor about getting on testosterone blockers. She'll be able to get counseling as well, since extensive counseling is a given when considering transition. The trans-friendly clinics/hospitals in our area have mental health professionals on staff and require their patients to undergo counseling before and during transition.