Is the partner obligated to guarantee the capital of his relative that was invested in the collapsed company? Is he legally obligated to return the capital due to the existence of a trust receipt? Does the relative have the right to sue him based on this receipt, given that the money was for investment, not a loan or a trust?

Since your relative gave you his money to invest in the company, and you did so, you are not a guarantor of his money. He is not permitted to use the trust receipt to pressure you and wrongfully seize your money, because an agent is a trustee, and a trustee is not liable unless he transgresses or neglects his duty. The conduct of the company's management should be examined to determine the guarantee.