Would you keep this abandoned dog?

Asked: Would you keep this abandoned dog?

Hi. I live on a ranch in a very rural area. Dogs are pretty much allowed to roam free, but since I don't have a lot of neighbors everyone knows each other's dogs, and if someone gets a new one they let it be known so it isn't shot (it happens where I live, no one freak out). Recently my little brother and sister had been going on and on about a "stray" dog. My parents went to check it out with them, and said they'd keep and eye on it. It'd been sticking near our place for about 2 weeks when they decided to do something about it. We called up people, and it took a while but found out who it belonged to (not a neighbor, someone a little further away). We drove the dog over to them, but they said they'd left it in our area on purpose and we could keep it. They refused to let us in, and wouldn't tell us anything about it's past or anything.

They live sort of in town, and we got a good look at their two other dogs, chained up in a small fenced in area and very skinny. We aren't going to call animal control because based on what we saw we couldn't prove they were being neglected. They had a few cattle who barely looked better. We decided we didn't want to leave the dog there, and brought her home. She's been with us for about a week now.

She's a Border Collie, and we could use another herding dog. We have one other, plus two hounds (pound puppies), but only the Collie herds. The thing is, my parents and I are very neglectant to introduce her to the herd. She's been very gentle and sweet around our other dogs, and they've already accepted her. I've been trying to train her with the little commands I can in our yard without taking her out to the cattle and goats.

My parents and I are just worried that she might turn viscious around them, and hurt them. We know nothing about her past, other than she probably wasn't treated well. What should we do? I had the idea of maybe bringing one or two goats up to our house and introducing them, but I still wouldn't see her in the herd. There's no way of singling out a cow or two, they're pretty much free range and not exactly tame enough to catch unless it's time to sell.

What would you do? Should we keep her? We thought about finding a home that we know she'd have a good family, since she's great around other dogs and my little siblings. But we also want to see how she reacts with the herd, because another herder would be a great help. I just don't want her to get out there and turn viscious, with no way of me getting her back. She knows to come, but my other dog is trained with whistle commands, which is how we get them to direct the herd. I can't exactly do that without putting her in the livestock.