What is the ruling on a sister contributing her money to the finishing and equipping of a commercial building for the purpose of renting it out, in exchange for her receiving a percentage of the monthly rental value for thirty leased months (with payments ceasing if the rental stops), with the agreement that she will not demand payment in case of loss, and then her capital will be returned after the agreed-upon period ends?

This transaction is impermissible because, in reality, it is a loan with interest. Naming things differently does not change their ruling.

If you wish for a permissible transaction, you can engage in a manufacturing contract (Istisna') with your sister, adhering to its Sharia guidelines. Alternatively, she can conduct a Tawarruq transaction with you, where she buys a commodity and then sells it to you at a profit, allowing you to benefit from its price.

As for the transaction mentioned, it is impermissible.