Boyfriend's nosy family

For the majority of the week, my boyfriend and I stay at his house with his family - His mom, sister, sister's husband, and sister's baby- plus us.

The baby, whom is almost 2, has a room.

My boyfriend and I share a room.

His sister and her husband share a room.

His mom also has a room, but she's hardly ever home.

Anyways.

In order to keep what I buy from being eaten, I keep it in our room.

We have a mini fridge in there, along with the family's deep freezer.. meaning, any meats, etc, that I buy must be in that.

Up until recently, his family wasn't in our room for anything except the deep freezer.

We work a lot, so I have a lot of quick meals in a box in his room.

Today his sister invited herself to come into our room, dig through the box and take half of the things in it for a meal for them last night.

We pay rent there, and we don't rummage through their things- so i'm looking for ideas to stop it.

I know she's not going to replace anything- we always bhave to fend for ourselves or would starve, but now she's taking from mine and my boyfriend's paychecks.

What can I do?

Also their little one comes alone messing in our room 24/7. He's figured out how to open the door, and it doesnt lock.