My siblings and I are expecting to pay enough towards the support of a married parent to be able to claim her as a dependent. Do we have to pay any service providers directly (landlord, doctors, car repair people) or can we give the money to the parent and let them pay the bills? If we do an electronic transfer of funds to a joint account can we still say that the money was directed to just one of the parents?

Thanks, M

You understand that your mother's spouse must file married filing separately.

There are several tests for dependency all outlined in http://www.irs.gov/pub/irs-pdf/p501.pdf
Your mother is your qualifying relative if she meets all the tests. Be sure to read the part about a "multiple support agreement".

I recommend against transferring money to a joint account. Joint means joint.
I would recommend paying bills by transferring the payment directly to those who are billing your mother although this may be troublesome.

Medical expenses for your mother are deductible for you if you itemize deductions.