A question that comes to mind is, are your dogs related? And your mother's dog, is she related? The fact that both pups had seizures within two days of each other makes me think they may have gotten into something poisonous. Is there anything they could have gotten into? Cleaners, something in the garage/basement? Did you have a good look around (you might want to get on all fours)? What are they eating? Where do you store it? Do you know exactly what tests your vet did?
you get to the bottom of this asap.