My sister and her fiance, James, and I were over at my mother's house visiting. Sometime during the visit, James tried to kill one of my mother's three cats with an ax. Somehow, we were able to get to her (the cat) in time to take her to the vet before she died. James acted like he didn't know what had come over him and as if he saw little wrong with his actions. The cat did live and had these weird plastic things on her chest that we were required to keep there while she healed. My sister and mother seemed to see little wrong with his behavior also and I was the only one who was upset at all.
Good morning, Zoe.
I think that the cat represents the relationship between your sister, yourself, and your mother. By marrying your sister, he is doing nothing abnormal or wrong, but it does weaken her other relationships somewhat. You shouldn't worry. Like the cat, her relationship with you and your mother will recover, even if it is strained at the moment.