Hi, Gwen.
Being transgender does not come from childhood abuse, or anything else in life. It is a feature we are born with. Options for getting rid of dysphoria are limited. Some people just live with it, and more power to them, but that doesn't seem to be working for you. For most, some degree of transition is necessary.
Have your family given any indication, via transphobic attitudes or statements, that they would reject you? You might want to start sounding out their attitudes.
Your stepmother sounds like she might be the one to approach first. She already knows that something is wrong and wants you to talk about it. You could sound her out by admitting that, yes, there is something bothering you, but that you are afraid to talk about it because you are afraid of the family's reaction. Seeing how she responds to that will give you an indication of how a full revelation would go.
When it comes right down to it, you have to do what is best for you. If you are dependent on your family, you might have to bite your tongue and say nothing for a while. If there is no dependency, then you have to eveluate whether it is worth sacrificing your own emotional health for the sake of your family's complacency. For some people, that is a valid trade-off. For many, it is not.
Above all, be strong.