The mother was shot in the face, and then the daughter turned the gun on the friend, at which point, according to the friend, the gun misfired three times. Perhaps it was divine intervention, since killing your kin should not spill over onto friends. But there's also a chance that the gun was to blame.
If you're planning on killing a family member, it makes sense to ensure you have a working gun. If you just stole it from a random glove compartment or inherited it from your grandpa, there's a chance the gun will malfunction. Therefore, before you plan your revenge for years of verbal abuse or whatnot, test the gun out. You'll be able to identify any problems with the gun before using it for its malicious intent.
One problem that can cause a misfire would be dud rounds. When I inherited a .22 rifle from my grandpa, along with about 50 rounds that he'd put together himself, and then left them sit for a decade, I'm sure there's a slight chance some of those bullets would have failed. I'm not a gun guy, so I don't know for sure, but if one wants to massacre his or her family, one should probably go out and buy some newer bullets with cash at a local pawn shop.
While I don't know if moisture really causes a misfire, it seems to make sense that it might. Even though you see it in movies, it's probably not a good idea to store your gun in a toilet tank to hide it, even if it means you always have easy access to the gun whenever Mom finally gets on your last nerve. Try storing it under a bed or on top of some cupboards instead.
But don't keep the gun waiting around too long before killing your annoying relatives. Guns will get dusty, and dust probably makes the gun more susceptible to misfiring. Again, I know nothing about the physics of it, but I do know that dust can mess up bathroom fans, and it's hard to remove, so I usually just douse them with lubricant spray until they stop making awful noises. Like door hinges, but that might not be dust. It's possible you should spray your weapon with WD-40 before you shoot at your parents. Or it's possible that's a bad idea. Google it first.
In Florida, it seems everything rusts faster. My kids' bikes are rusting, my grill grates rusted through, and there's rust covering the lighting in our bathrooms. While it might be that the humidity causes the premature rusting, it's also possible that I have an old grill and cheap bikes/bathroom lighting. Maybe you got yourself a cheap gun. Something made with substandard Chinese steel that rusts more readily, especially if hidden in a toilet tank. Actually, the hinges are probably rusting, too. In fact, spray the hinges in your family member's house while you're inspecting your gun for rust. Squeaky hinges have ruined plenty of family murder plots, but it could add a sense of dread, so leave them squeaky if that's the intent.
There are apparently other mechanical issues that can happen with a gun, and maybe you should have it inspected by a professional before you off your uncle. He deserves your best effort. After you get it inspected and cleaned, take the gun to a shooting range or a local wooded area for some target practice. Remember, unless you're planning on shooting your family while they sleep, they could be moving, so practice capping some squirrels or geese rather than just the trees.
Killing your family is a big decision. Choosing the right gun will help take some of the stress out of your murder spree. Good luck, and have fun.