She is so skinny

My little girl Avalon is so skinny. When I pet her, I can almost feel her skeleton. Particularly where her leg bones meet her spine. Her twin sister seems to be a fine size for their four months. Avalon eats okay. They both love the one can of wet food I give them. They go through water fine. Their feces has had a strong smell the last few days, but, judging from the amount, they are both GOING. I am just so lost. Their first vet appointment is not until June. Could it just be my overprotective nature?

She could have worms.  I'd move the vet appt up to as soon as you can get in and have her checked out.  Also, what are you feeding and how often?  Do you think she's being antagonized and kept from eating enough?  Do you feed canned or dry or both?