Barcaman,
I am sorry to hear about your situation. I am not an expert on the family matter but I would like to share my thoughts. First of all, what do the cousins do? I meant are they student? Based on your descriptions they are either students or lazy bumps. If they dont work and don't really do anything, approach them and let them know what you think. If they are students, I would be considerate. We are brought up on a society where one relative cares another. You can consider it as a favor for him/her. Do you rent more space just because of the cousins? If not, I would not worry much. About groceries, its usually not that much difference if you shop for 2 or 3 persons. 

You should be having this discussion with your wife than here in sajha. No matter she likes or not, its what you think and you have to let her know. You can't let this thing bug you and create a rift between you and your wife. Who knows the cousin or his/her parents have helped a lot to your wife n her family and she feels obliged? The point is you need to know the feelings and thoughts of your wife in this matter. Talk to her coz you both have share the same house and belongs to both of you.

Good luck man and get it resolved!