Help! My babies keep dying!

I swear my biggest chick is doing something to the others! So originally I got 4 chicks....3 EE and 1 Rhode Island (they were 1-2 weeks old from a feed store.) Temperature in the Brooder is fine, water and food aplenty, etc. 2 days later 2 were dead, just acted listless and then just dead. Then I went to a different feed store, got 2 more and then the same thing...2 more just dead. The funny thing is, the original 2 of the first 4 have become best friends and they are both fine, it's just the 2 replacements that keep dying! Any suggestions about what is going on? It's my first time being a chick Mama and I'm just about ready to give up. My kids don't even want to look at the chicks anymore because the new ones keep dying and we are very suscpicious of the first two! Help.

 

Could she seriously be doing something to them?