I don't think there is anyway for your sister's family to see both parks unless they buy two 1-day tickets. That will cost about $102 for each adult and $81 for each child 3-9.
Do they know anyone who has park hopper tickets with days left on them? Or can they buy hopper passes and save the days for another trip? Or find someone they know to share the hopper tickets with (but not on the same trip)? I've seen partially used hopper tickets for sale on ebay, but I don't recommend this method as you wouldn't really know what you were getting until you actually tried to use the tickets
Even if they were staying on site, the shortest UPH ticket is 1 night/2days and it costs more than 2 single day tickets.
Can't think of any other options. Hope you figure it out.
Edited to add:
They might be able to get a 2 day/2 park ticket. I think these are the tickets that used to be called "Bounce Back" tickets. Accoriding to Deb Wills' WDWIG:
"2 Day 2 Park- Adults $96.46 (includes tax) child, $75.26 (includes tax) expires 4 days from *purchase*. (1/01)
Here's a link to some FAQ on tickets from the WDWIG:
http://wdwig.com/ticket.htm
and here's a link to ticket info (with prices) from the DIS:
http://www.wdwinfo.com/wdwinfo/tickets.htm#UMP Prices for 2001
Maybe one of them will give you some other ideas. Good luck!
