Is it obligatory for me to return the 'āniyah (gift given on the occasion of a wedding) that my divorced sister-in-law gave me four years ago, especially since I did not borrow it from her and I am currently experiencing financial hardship in preparation for marriage?

If the wife's sister was an adult and of sound mind at the time of the gift, then she is not allowed to retract what she gifted or gave in charity, due to the hadith: "The one who retracts his gift is like the one who retracts his vomit," and "Indeed, the one who retracts his charity is like a dog that returns to its vomit." However, if she claims that she gave it to you as a loan, and you claim that it was a gift, then her word is to be accepted, and you must return the amount to her.