I can so relate to this situation. As I have lived with this my whole life, my father is a person very much like your partners sister very moody and very vocal if things dont go his way. Our family have to tip toe around him all the time, I'v tried moving out three times but had to come back through financial problems in the end, so till this day, I still go through his tantrums daily.... It is so tiring and my poor mum her self esteem is so low through having to take this daily too. I love him very much, but dont like the way he acts sometimes and this has made me, very sensitive to other people's moods. I feel for you at the moment as you have gone there with your partner to enjoy yourselves, I think you and your partner should sneak off when ever you can for some TLC together and let her get on with it