So let me get this straight. Being trans isnt a social contagion but the odds of two in the same family being trans are very low indeed.
Teenagers who are the opposite sex shouldn't share a bedroom, but if you 'change sex' this is somehow still ok to your adult child because it allows them to not have to take responsibility for themselves and go and pay their own way. Can you imagine this in another household where a 13 year old girl has to share with an 18 year old?
You moved to a smaller house on the basis that they could share and the elder one 'would grow out of it' but this hasnt happened and now they are an adult they need to face that.
You should at show some consistency. If you are supporting your elder child's identity they need to move out because there is no room for them in the house, now they are an adult.
Otherwise its your houserules and the internet is binned and they are not to talk to their sister about this nonsense. Put your foot down.
You have to ask yourself can you save both kids from this nonsense or will you lose them both to it? Is it better to cut the social contagion out and ask the elder one to start pulling their own weight and taking responsibility for themselves?
It sounds a lot like you are facilitating the problem at home by allowing them to have their cake and eat it by indulging the fact shes female when it suits her because she can then sit on her arse at home on the internet filling her sisters head with nonsense rather than be a grown up.
Honestly, you are being walked all over here by 'being kind' and im not sure its good for either child.
The older one has to make a choice. Is she male? And if so she moves out. Or is she taking the piss and playing fantasy land because shes still not emotionally mature enough to look after herself? She cant have it both ways.
No easy way out of this. I'd be trying to give the younger one a chance and spelling out to the older one the consequences of being an adult and making adult decisions.
You are respecting the elder ones adult identity and choices in doing so.