I see that you live in Vermont from your profile, and I am in Mass. However, there are 2 places here that do sleep consults. One I know they do over the phone, the other place I am not sure. But here is the info!
http://www.jfcsboston.org/fcs/sleep_consultations.cfm
Check out their web-site and either call or fill out the form to get an apptmt set up.
The other is the Sleep Center run by Dr. Ferber himself: ###-###-####.
Otherwise, here is what I would suggest:
I would walk in and walk out a few times for your daughter, check in on her, tell her you love her, check under the bed, say there is nothing to be afraid of, give her a special blanket that will 'protect her', then leave - for the night. No more water, no more PJ's, that's it. Period. She now knows her screams will get you to go back in.
Say - good night, see you in the morning and mean it. Once you do this and do NOT go back in, she will learn fast. Make sure you praise her in the morning!
Can you cut her nap down to one hour instead of 90 min?
Now so she will not wake up your son - put a fan and a humidifier in your his room to drown out her screams. One on either side of his crib so he can't hear her.
Good luck!!!