I agree with the opinions expressed about not placing her in another home and making sure to get her temperament professionally evaluated...if her temperament is indeed questionable the two options of managing her very carefully or euthanasia are the only responsible options.
On the other hand, there are so many questions to be answered - please don't make up your mind until you know the facts. You've stated that:
1. The event occurred on YOUR front porch.?..what was your neighbor doing on your front porch? This doesn't excuse the biting, but it is weird.
and bite my neighbor and his dog
The neighbor AND the neighbor's dog were bitten...did the neighbor's dog run up onto YOUR porch, then get in a scrap? Maybe the neighbor was bitten while breaking up the fight? This is very different than your old dog attacking the neighbor and would still require a quarantine.
3. Is it your westie or your pit bull who is
old, has had cancer and has arthritis and is going blind
Not that this excuses biting, but it adds another dimension to the incident.
4. Was your house locked? Was your old dog traumatized by a break in? How in the world did they get out?
5. Animal control hasn't deemed her aggressive...they MUST know something you don't - and it does sound a least a little positive.
These are not meant as any criticism to anyone, and ultimately you must act responsibly. But please don't buy into the pit bull stereotypes until you have all the information. She sounds like she's been a great companion for 8 years....