For the sake of family harmony, you should give back to Cuz1. It isn't your decision to make as a non-blood family member. This should be between her and her brother. Did Cuz2 assist in the care of the mother or was it all on Cuz1?
People become sentimental and reflective after losing a parent. You said yourself that she cared for her mother before she died. She could have been overwhelmed after caring for her mom and dealing with the death. I think it would be really mean for you to hold onto the stuff. It really isn't your decision to make... your cousin could have still be in grief when she gave you the box.
Sure, you could pull the 'she gave it to me, so it isn't hers anymore' card, but do you really want to tarnish family relations over something sentimental to her? It is not our place to determine that and the kind thing to do is give her back the family momentos.
You have misinterpreted the situation quite a bit. I'm offended at being told it would be mean of me to do something that I quite clearly have no intention of doing. It is indeed my decision to make as to which cousin gets the original documents for their family, and it's a sticky situation, which is why I posted here.
I have not pulled the "she gave it to me, so it isn't hers anymore" card. I don't know where you got that idea.
My "place" is to sort through the boxes of documents, to sort and catalog them, and to pass them on to their rightful owners, which has to be determined by me. It will take a huge amount of time, effort, and expense to do this, but I'm happy to do it because I consider the documents to be an invaluable treasure.
An, as has been pointed out, Aunt did not die. She's in a nursing home.
You are misinterpreting and taking offense when none was intended.
They are not your documents to decide. This is a sentimental family issue and as others stated, should be duked out by the siblings. The pulling the card statement was not meant as you took it. Here at Ehell, we discuss all the time that once a gift is received, it is 'yours' to do with as you wish. However, given the sentimentality and familial harmony issue here, it would be mean for you to hold onto them. Whomever you decide to give them to, the other will resent you.
As you stated they are THEIR family documents. Not yours. Given the stress of Cuz1 dealing with her ailing mom (sorry, I did get that part wrong... reading on a phone is difficult at times), I can see why she made a rash decision giving them to you.
The point is, even if Cuz2 is interested in geneaology, it is not your decision, even if Cuz1 gave them to you. The two siblings should discuss and decide. Otherwise, no matter what you choose, you will be the bad guy to the non-chosen one.