
Quoting
harrylime
1) In the typical living trust, the owner(s) of the property create the living trust and transfer property to the trust. The creator(s) of the trust generally remains the trustee(s) while living. The trust document establishes who the beneficiaries will be when the creator(s) of the trust pass away. It would be an unusual situation where "... as time goes by the ownership of the house slowly transfers from the parents to my wife and her sister." That would mean executing deeds every so often to adjust the ownership interests of the living trust, you wife, and her sister. Not very likely. So, check the county records for the latest deed.
2) Now that you have explained you wife's sister's situation, you introduce another complication. For your wife to sell her interest, there needs to be a willing and competent seller. Where would the funds for the buy-out come from? The trust established for your sister-in-law? The trustee has a duty to look out for your SIL's best interests. That would mean evaluating whether a buy-out makes sense and, if it does, to negotiate the best terms possible.
3) If you and your wife enter into a guardianship, you need to be careful about any conflicts between your SIL's interests and your interests.