I have three cats. They were born of the same litter and all are
identical.  If they were human they would be identical triplets whom
look exactly like there mother ( color and markings ).  Their mother
died when they were just weeks old so my husband and I raised them..
There are two males, one female.  My question is that now they are 8
months old. The boys are both 11 pounds but the girl is only 6.6 pounds.
Should I be concerned?
They are all happy and healthy and she eats as much as the boys.  I
don't think they know they are cats really since they never had an adult
cat to teach them anything. They stay outside all day. Our yard is not
fenced in but they never leave the back yard. Just like a child that was
taught that. We always took them out there when they were young but we
never left the back yard with them and now they never leave it, thank
goodness for that is how their mother died.. They do normal cat things
like catch moles and mice and bugs etc.. outside tho. They come in at 7
at night , have their dinner, run around and play with us for a couple
hours and they all share a bed. They sleep all night and never use a
litterbox ( tho we have one in the room ). At 7 am we let them out of
the room, feed them, then they  crawl in our bed and snuggle  like a kid
would with his parents. We leave them out around 9 and start the day
over. So they are healthy but she is so little.