I'm there with my parents. It's very hard. My parents are 67 years old - young, really, but my dad has early-stage Alzheimer's and my mom had a car accident (and head injury) 9 years ago that makes her memory and problem solving skills bad. They live in our childhood home, go through the motions like they are normal, functioning adults, but they've completely screwed up their finances, drink way too much, need guidance for so many things (like with their TV, internet, cars, prescriptions, everything), and just generally need us to orchestrate their lives.
My dad is at a 30 day rehab center right now for alcoholism and severe depression (suicidal). We told him he had to go and told my mom there was no other option. She is on board now, but it wasn't easy. My sister already handles their finances, although she hasn't gone as far as limiting their access to their accounts yet...that will probably happen eventually.
We never sat down and said we were taking over, but we've slowly worked our way in to "help" them and eventually started taking care of things.
In your parents' case...does one of them have an illness that makes them incapable or have they always been that way? Hugs to you....it's a very hard place to be.