Is it more befitting to pay off the interest-based loan taken from my partner brother, or is it more befitting to use the remaining amount for marriage and to buy an apartment, knowing that this amount represents everything the questioner owns?

What is legally binding upon you both is the repayment of the principal amount of the loan, without the interest. This is because the loan was a فاسد (corrupt/invalid) contract. The fundamental principle regarding فاسد contracts is that the lender is only entitled to their principal amount, as Allah Almighty says: (And if you repent, you will have your principal – [thus] you do not wrong, nor are you wronged). You can combine repaying the loan and getting married by marrying to protect your chastity without extravagance in expenses, and by trying to save a monthly installment to pay off the debt, while seeking Allah's help.