My mother in-law lived with my wife and I for 6 months and paid no bills or rent can I claim her as a dependent on my taxes? And if so what form would I use.

Maybe. The rule is that you contributed more than half of her support for the entire year. If you do claim her, she has to mark that she can be claimed on someone else's tax return as a dependent.

I'm unsure about the form. I think you can claim her on any form -- for sure any form where you can list your dependents and their social security numbers (and for sure on the long form 1040).

Does your mother has SSN?

Mother inlaw has ssn and has income from ssi but does not file. Can she still be claimed? If so which form or forms?

You can claim your parent as dependent (qualifying relative) if
1. You provided more than half of their support, and
2. the parent did not earn more than $3,500 in 2008.
It is not required that the parent live with you. Read about Qualifying Relative: : Your U.S. Tax Return: Requirements for claiming a dependent

Her SS income counts toward support but not in $3500 income.

Given the amount people get from Social Security, it is almost a sure bet that you provided more than half of her support.

Claim her.