Perhaps you could suggest they babysit at your boyfriend's mothers instead. If it is her house, then there would be more rules for the 17 year old niece and you wouldn't have to worry about what she was getting into. Just pack enough food and diapers for the day.
You might try talking to your boyfriends mother and tell her that you've noticed that on the days she is babysitting, your things have been gone through and used (your makeup and such). Tell her you would prefer not to share those items. It is a health consideration. Women are not supposed to share their makeup because of shared germs and such. Also, try leaving a note on the refrigerator that says, please don't eat the following items but also add on there to help themselves to the rest. In this way, they will understand that you bought those foods special for your diet but you appreciate what they are doing for you. You may, also, want to leave a treat that you know they like, such as cheesecake. When they get there, tell them you left it in the refrigerator for them because you appreciate their help. Say it all in a kind tone and follow it up with sound reasoning. In this way, you can set boundaries without making them feel unwelcome.
If that doesn't work, you could look into another babysitter. Perhaps you can find someone else who works part-time and trade off on babysitting. Check with friends and neighbors on who they would recommend for sitting.
Good luck.