Join Date: Oct 2009
Location: Just south of sanity
Yes, it's possible for a 27 y/o mare to be pregnant but pretty unlikely, especially if she was extremely underweight. Was there an intact male in that field with her?
What makes you or your sister believe that the mare is producing colostrum? Do either of you know what healthy colostrum looks like? The mare may simply have mastistis and is not actually producing colostrum. The appearance of 'milk veins' do not necessarily mean the mare is in foal.
At 27 y/o, thin, and in obviously bad health prior to coming to you, her chances of having a healthy foal are very bad if she is indeed in foal.
Instead of relying on the internet and a bunch of random strangers, you need to get a vet out ASAP to determine just what is going on with her.
Good luck, and tell us what the vet has to say when you find out.