I am also sorry for their death. I agree in the poison theory. There are so many common things in a home that are highly poisonous ot cats. Onions for example! Here's a link to a website about basic cat care, and at the bottom is an extensive list of things that are poisonous to cats:
cat care

Its sad to hear that two beautiful kittens had such a short life. I hope that you can determine what happened and give yourself some peace and closure.