Hi Leslie,
I'm sorry I didn't see this post earlier. I checked four sources for the toxicity of mistletoe to sheep. It is not mentioned in Poisonous Plants of the Midwest (Univeristy of Illinois, 1972) nor in the Sheep Production Handbook (2002 ed, vol. 7). Laura Lawson in her book, Managing Your Ewe, doesn't address poisonous plants at all. Paula Simmons and Carol Ekarius, state in their book Storey's Guide to Raising Sheep, that some plants poisonous to cattle and horses don't affect sheep at all. So unless you have the one-in-a-million ram or ewes they should be okay.
Karen