Fleas?

I'm not sure where to ask about this one so move if nessary mods, My sister has a little dog which she uses that flea away stuff on regularly,the dog doesn't seem to scratch at all. Yet all dwellers in the house are being bitten regularly,including the odd visitor,me. I was bitten 3 times on the leg,I was only in the house for 30 minutes last week. She doesn't have carpets in the house where the dog has access and she removed a rug that she suspected was home to something or other,they also have leather seats,no cloth. They have scoured the house but have seen no signs of anything. I know a girl who lived in a house infested with fleas from a cat but you could see them,in my sisters they haven't seen anything. Any idea's or suggestions?