jrh
I am so sorry for your pain. She really is one sad case and needs help but unless she is willing to get it there is not much you can do.
This may sound cold, but you have a lot of years left and deserve to be happy. You are not responsible for her and her choices, and sadly her choices are self-destructive whether from alcohol or guns. You don't have to go down with her.
My suggestion would be to talk to her doctor and see if she can be committed since she is threatening suicide (in Canada that would be grounds). Or ask of he has any suggestions. Do what you can and then look after yourself.
And perhaps remove the gun from the house. If they can find booze that we hide, she can find the gun.
And stick around, read anything that will help you, and know that we're here and we care.
Hugs
Ann