How to live with a transgender child

[jds124]

My niece and nephew have lived with us since they were 14, today they turn 18.
My niece was in therapy when she first came to live with us. She was treated for sexual abuse, by her half brother, self harm, anxiety, depression and we added grief therapy when her mother died.
We additionally had her tested and diagnosed with high functioning autism.
She informed us that she was lesbian when she first moved in. I often wondered if her choice had to do with her sexual abuse.
Earlier this year I received a call from the school counselor, my niece doesn’t think she is gay, but transgender.
She has since cut her hair, binds her chest, and has asked her friends and teachers to call her by a boys name.
I still think part of this is due to the  sexual abuse. I’ve also noticed with her autism that she fixates on things.
My husband refuses to talk to her about this. But sure has a lot to say to me, about how this and that will not be allowed in our house.
She wants to go to counseling and eventually transition. She has only told one set of grandparents, who live in another state. They seem to be ok with this.
I told her if she wants to pursue this further she will have to make it happen. I cannot continue to take off work to take her to appointments. Then my husband tells me, she can’t transition while she lives with us.
She’s still in high school, and will probably be dependent on us for longer than most young adults.
I feel stuck in the middle. She just wants to be accepted.
I still refer to her as my niece and call her by her birth name. I haven’t accepted this, nor wrapped my head around it.
I don’t want to hurt my marriage, nor disregard my husbands wishes. But I also don’t want to hurt my niece.
I also feel, that until she tells other Family and friends it will be hard for me to acknowledge her as a boy.
When she and her brother moved in, I was thought raising two more kids wouldn’t be so hard. I had already raised 3. I figured 4 years of high school, then off to college. I had no idea of the abuse, mental issues, etc.
I just don’t know if I have it in me to deal with this. Especially when I feel like my husband had checked out and left all parenting to me. We are starting to fight over things. And these are his sisters kids, so I don’t think it is too much to ask for his help.
How would you handle this?