When is a "skinny" cat too thin?

Here's a little background. We have a 9yr old blue point siamese, her name is Taxicat, we also have her sister. Also have 4 more kitties (sisters age 6 and sisters age 1). All fixed. She is clearly the 'alpha' kitty. She became quite 'chunky' when we adopted the 6 yr olds, probably from eating their "kitten" food but gradually lost the weight. She did not eat the 1 yr olds kitten food.
They are all fed Nutro dry food, bowl always filled. They all love their morning "kitty treats". Except for Taxi, the rest of the girls are quite "matronly".
We have noticed lately that she seems to be much much thinner No ribs showing but her hip area (part of body in front of upper back legs) is very 'caved' in and she feels light as a feather.
Any ideas?