Suddenly now it's her daughter with MH problems.
I'm so sorry OP, but I absolutely believe you are being played.
Her talk of taking on the morgage is confirmation that she is planning for you to no longer reside there.
Two weeks for her to seek legal advice and to frame this.
OP, all we can do is give you an alternative prospective to help you protect yourself.
You have to want to protect yourself.
You have to want to not be taken advantage of.
You have to want to not look back on this an know that you had another narrative to consider, but just went ahead, against your own best interests.
You not returning home because of your new step daughters sudden MH problens, which have suddenly developed after your new wife's suddenly needing space is just you being played for a mug, despite that you are telling her you won't be a mug.
Get legal advice ASAP and help yourself.