How do tell or explain to your four year old why his cousin no longer lives with him?

My nephew and sister have been living in the same house as my parents, my son and I. Yes that is a lot of people in one house. In trying to keep this as simple as possible my sister and I were both told after our sons were born that if we got pregnant again before we moved out of the house we would be kicked out. No questions asked.

My son and nephew have more or less been raised as brothers rather than cousins. My son is four his cousin is two. I came home today from being out all day with my son to find out that my sister and her son no longer live with us. She got pregnant again.

My question is how do I tell my son that his cousin will no longer be living with us?