Transfer of property ownership; interesting situation

I have a somewhat novel predicament.

My sister and her ex-partner commenced a small development project with a house and land package. During the construction of the project they ended their relationship. Legally, ownership is 50/50. The Ex wants out of the arrangement, and mutually agreeable terms have been reached between the sister and ex. So, the Ex will have a no holdings in the development and will be compensated as agreed.

Due to our familial investment setup, parents are guarantors of the loan for the development project. Now, the change of circumstances means parents want to come aboard on the project. There has however been a complication in transferring ownership from my sister and her Ex to my sister and parents.

Refinancing has been preapproved (equity and loan serviceability is no drama), on the condition the builder writes up a new contract with my sister and parents names on, rather than my sister and her ex. The builder will not write this new contract, as building progress will halter as the plans and following documentation will have to be resubmitted to council for reapproval.

The secondary option is to keep the finance under the sisters and ex's names until property is complete (almost finished), and then refinance. This means transferring ownership on a home. What are the stamp duty implications of this? Sisters ownership will stay constant, on the ex is substituted out for my parents. They are already guarantors.

Some advice and alternatives if possible would be appreciated. Cheers :D

Edit: Apologies if this should be in Accounting / Tax, but I thought it was more of a hybrid question.
Depending on the relevant state laws, if it a breakup, then a stamp duty exemption may be allowed, meaning you could complete the project and not be hit with stamp duty.

Something to look into.
My understanding is the parents will be liable for stamp duty, calculated on the value of the property when they purchase it. Only if the share of the property were to be transfered to one of the spouses would there be a stamp duty exemption, the so called love and affection rule.
Messy situation. Probably best to wait till it is finished if it is nearly complete. Either way you will have stamp duty, you may save a bit by doing it before the project is complete, but you will have more problems with the builder etc which will mean more costs and more delays (and interest).

Stamp duty exemptions are usually on for transfer between spouses. Don't think it will apply for a partent. Being guarantors will have no bearing on stamp duty.

If only you had a corporate trustee you wouldn't have had issues.