Must I disclose the $350 I found in a garment belonging to my siblings after a month has passed without anyone asking about it?

You were negligent in not paying the amount to the owner of the garment in which you found it. What is now obligatory upon you is to hand it over to him because he is its owner. This amount is not a 'luqata' (found item); for money found in a privately owned place belongs to the owner of that place. If he does not claim it, then it is given the ruling of a 'luqata'. Not asking the owner of the garment or his brothers about what was lost does not justify not informing them about it.