I'm sorry this is causing you more grief. I can imagine it's like losing both parents, because it's difficult to recognise the person you know as your mother.
I think we forget sometimes too, that our mother had an entirely different life before marriage and children. So no, you are not being unreasonable.
It may be she's somewhat of that generation that you need to be attached to a man to feel valid. Or it could be that she feels completely lost and terrified without your father, and she's trying to fix that. Could be she's determined not to be alone, so anyone will do. It could just be for the sex. Perhaps before marriage she was a wild child but settled down for your father and you all, and now she can return to this. Perhaps she's letting loose after being a carer and watching her spouse die. Could be that she's trying to do all this for herself, but not in a very measured, conscious or elegant fashion.
Also if she's of retirement age, it's competitive out there for that age group as it's still a case that more women then men are available. Or perhaps it's been such a long time since she dated (if ever prior to your father) that she's trying everything.
If you want to make sure she is OK (and be there for her), you may need to temporarily put aside your feelings, and your needs. I would encourage you and your brothers (what are they experiencing or what do they think?) to actively maintain contact and communication with her. Keep those lines open. When you mentioned certain behaviours, like impulsivity, and spending, also lying and getting nasty when confronted - was she always like this? All that behaviour was kept under control by your father perhaps?
However, if this is new behaviour, something else might be going on. So perhaps it's important for you and your brothers to somehow endeavour to understand her further, in a non-confrontational way. Engage her in discussions, ask qeustions so you can understand more. BTW, have you and your brothers had any discussions about this?