Wow, it sounds like you are going through a lot right now. I am so sorry for your sister's passing
I'm glad to hear that your vet is going to provide some of his services for free to you. I would get here there as soon as you can to get an ultrasound done. You need to know if she really is pregnant and if so, how far along and how many pups. It might even be safest to just do an emergency spay. I would talk to your vet about it.
Thank you for taking these two poor souls in. They must be completely lost without their owner.