I recently adopted three abandoned kittens (two months old). They are now about two in a half months and are healthy as can be, growing up fast. The only odd thing about them is that the female likes to suckle on the largest of the two boys, just behind his right shoulder. They're laying in my lap at the moment, the girl is just suckling away and the boy is grooming her head xD
I googled it and turns out it's rather common in kittens separated or weaned to early from the mother. Theres no harm in it and most do outgrow it, I was slightly alarmed when I first saw her doing it.
I have to say, the number one problem caused by a kitten suckling on another is that the suckled smells like wet cat drool
They both seem rather relaxed and happy, so I'll just let nature take it's course and either she'll grow out of it or I'll have a slightly odd cat for the next 15-odd-years >-<