Oh, leave it to Tater...
Anywho, if you are concerned about your parents financial well-being, ask what they need and why. If they don't want to answer, then that is their problem. However, they opened the door by disclosing that they need money, so it IS your business now.
As for the firearms, get them back in your posession asap. No matter what paperwork you may have, posession is 9/10s of the law. If you ever went to court over the matter that question would come up; who had them and for how long? I don't think you want any part of suing your brother-in-law or parents for a couple of guns that could be valued at less than $2000. So reclaiming them is the best thing to do.
I know what it feels like to want to avoid conflict and be the peacemaker, but don't let yourself get shit on...you are already getting that a plenty right now. You don't deserve it either. Be good to yourself, if others - even spouses or parents can't get on that train, then stand up for yourself (like you did by telling your mom to slow down with this dash for cash idea of theirs).