Psychic photography though rare is not unheard of.
It sounds like you know for sure that the camera was never at your sisters house.
Please talk to your husband and anyone else who may have access to the camera and ask them to see if they borrowed your camera. Also, since you only visited your sister in-laws house once, are you sure the pictures are from her house. Have you asked your husband to take a look at them.
Please, we are not doubting that your pictures are paranormal in nature. To be sure though, you need to eliminate the normal possibilities.
It may have been your husband impressing the film with pictures or another relative at the party impressing images onto the film. More excitingly it may be your sister in-law impressing the film. She maybe trying to get a message of some sort to you. Something maybe to do with the house. Like something hidden in the house.
![[Eek!]](eek.gif)