Help! One chicken died and now another is acting sick.

[Katieswan]

I am really struggling to figure out what is going on with my chickens. Around a month ago I got two buff orpintons that were probably 2 or 3 months old. Supposedly they were siblings of another one I got a few weeks before them. Suddenly one of the new buff orpintons started acting ill. I thought she got pecked in the eye because her eye was shut and kind of swollen. She was acting fine and then a few days later I noticed she was not eating much and acting lethargic. A couple days later she died. I did not see much for other symptoms except for occasional diarrhea. The day she died she seemed to have watery eyes and mouth as well as the closed eye. I thought maybe she had an sinus infection or something. Now, the second new bird is acting lethargic and not eating much just a couple weeks later. She does not seem to have any super apparent symptoms. The only weird thing is that both of these birds seemed small and not growing very fast compared to the other bird I got before them but was supposed to be there sibling. This buff is about half the size of the original one I got. All three birds are in the same coop but the first larger buff seems just fine and is growing normal. Any ideas on what this is? could it be worms? What would the symptoms be? I am feeling at a loss since I tried to figure it out for the other one too but didn't have any luck. Thanks in advance!