All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It appears from what you mentioned in the question that the gold was a deposit with your brother in-law and it was stolen from his house.
If this is the case, then he is entrusted and he does not guarantee (the loss) according to the view of most scholars unless he has transgressed or was negligent. Transgression means doing what is prohibited, and negligence means refraining from doing what is obligatory.
Amr ibn Shu'ayb narrated from his father that his grandfather said: The Prophet said: “There is no guarantee on the person who is entrusted (to store an item with him).” [Al-Bayhaqi in his Sunan]
Ibn Qudaamah said in his book Al-Mughni:
“A deposit is a trust, if it is ruined (or lost) from the person with whom it is entrusted without any negligence on his part, then he does not guarantee it, whether or not he lost some of his [the person with whom it is deposited] own money. This is the view of most scholars. This was narrated from Abu Bakr, ‘Ali and Ibn Mas'ood . This is the view adopted by Shareeh, An-Nakha'i, Maalik, Abu Az-Zinnad, Ath-Thawri, Al-Awzaa'i, Ash-Shaafi'i and the scholars of the Hanafi School of jurisprudence.” [End of quote]
Therefore, if your brother in-law did not transgress or was negligent in guarding the deposit, and it was stolen from him as well as his own money, then he is not obliged to compensate you for the jewelry or anything else that was stolen from him. However, if he wants to donate to you, to help you out, then there is no harm in accepting his help if he is solvent.
But if he is not solvent, then it is more appropriate not to accept that from him while thanking him for his intention and supplicating Allah to bless him and his wealth, so as not to overburden him with something that is beyond his capacity, even if he is happy to donate to you.
Allah knows best.