How old are they? Are they spayed/neutered? If so, at what age?
Sinister does seem a little heavy (not by much tho), but looks very well muscled. From the pictures, Athena looks the correct "weight" but seems to have very little muscleing, which can make a thin dog looks A LOT thinner than they actually are.
So just wondering if she is young and has yet to "fill out". But even then, my 21" at the withers 58lb female has way more muscle than your girl, even as a young dog her front end was well defined, just petite.
I would say you could increase her weight as long as her exercise routine allows for that to be turned into muscle and not just adding fat to her mid-section. If her shoulders and rear had more muscle I would say her mid-section would be almost perfect