@kartwk A few considerations:
Depending upon the state you live in, once separate property or assets are "commingled" - as when a solely owned home is sold, and the proceeds used to purchase a jointly owned home, it becomes a "marital asset" not a separate one. In most states, your step-daughter has NO claim on marital joint property while you are alive. You need an attorney (for yourself) to protect yourself according to state laws since she is trying to coerce her Dad. Please don't rely on the internet or "armchair lawyers" for advice.
While consulting the attorney, you may want to discuss having him draft reciprocal powers of attorney for both of you "in case one becomes incapacitated."