I agree with both of you, as that is something I experienced with my 14 year old Jack Russell some years back.
Your poor lovely old dog! No-one can make you have her put to sleep. As long as you are sure she is still ok with life, and still has the will to live, and isn't suffering uncontrollable pain. Go get her treated, as Ghilly said. There may be many solutions available yet for her problems. Just because she is 12 doesn't have to mean she is at death's door yet.
And give her a couple of hugs from me!!
I agree with Ghilly. It is very hard but as Ghilly said she will let you know.
Awww, poor old girl. It's so sad to see them get old.
You may want to take her to the vet to have her checked out, because the swelling could be caused by kidney failure or congestive heart failure. Something is making her retain fluids, and heart and kidneys are the two things that come to mind first.
As far as her being thin, some dogs will drop dramatic amounts of weight. It's the body's way of getting rid of extra ballast. This way, less food is necessary for maintenance. As long as she appears to be comfortable, I would let her go on for a while. When she's ready, she will let you know, but you will see it on your own. I do think she needs to be checked to get a handle on that swelling, though.
Please post back and let us kno how she's doing. I have a real soft spot in my heart for the senior dogs.
Ghilly