So background information. My sister, who is currently 16, has been living with me for the past couple years after some problems with our Mother arose. This past May we moved off base into a home with a work friend (he is 27), who was recently divorced and struggling to pay for his large home. In the home we all have our own rooms, separate living area like rooms, and bathrooms. We really only share the kitchen, but not food.
Since living with him relations between him and my sister have gotten quite bad. They argue constantly and can't be in the same room with each other without getting progressively more aggressive. When I'm alone with one or the other of them, they complain constantly about what the other has done recently. I'm worried without a way to mediate between them, my sister and I will have to move in order for her to keep her sanity. She has enough to stress about without all that house drama.
So far my advice to her was to completely avoid him when she is upset, and to instantly leave any room/conversation with him if they start to get heated. This is just avoidance though, and not a good long term fix.
My question for you all is: what can I do to mediate between these two people in order for our living arrangement to return to a more peaceful state?
Also important note, that the roommate and I are decent friends. He is never short with me, and we've only ever argued after a conversation between him and my sister went too far south - which he listened to, apologized to her, and then proceeded to sulk around the house for 2 weeks making everyone miserable.
Edit for questions: The living arrangement is expected to end in this coming July (about 6 months). We were only living there until we could find a more permanent home, and he is planning on retiring from the military in July and moving states.
Also the most common argument is about her not bringing her house key with her when she goes out, to martial arts or to a friends house. She will ring the door bell, his tiny dogs will lose their mind, and he will stomp his way to the door and let her in. This leads to him being gruff with her, and her losing her temper and being rude to him. My room is quite far from the front door, and unless she has the forethought to call me when she gets dropped off I will not make it there before him. Another common argument is about her not wanting to deal with his dogs, and not letting them out of their crates when she gets home. This upsets him and they argue about whose responsibility that is, and in this situation I think he is more in the right since they are living beings and it doesn't take much of her energy to put them outside when she gets home. So in short, they can both be the 'childish' one in these arguments - depending on what started the confrontation.