I have been diagnosed with the same condition, it's medical term is 'unilateral renal agenesis', basically make sure your sister takes on board the following lifestyle changes in order to help her with her condition (as for the increase of the kidney, i can only advice you to speak to her gp or kidney specialist about that my friend): she can live a normal, regular lifestyle but she will half to take note of her salt, protein intake...

. invest in a MedicalID pendant/bracelet listing her condition(s), allergies and medicines.
. has her blood pressure checked atleast once a month if her b/p is stablised with medication, or once a week until her gp finds the correct dosage to control her b/p.
. has her urine tested atleast once every six months to check her kidney, liver etc. function.
. eats plenty of fresh fruit, vegetables, fish etc. (a food chart can be found at the british hypertension society).
. drinks plenty of water.
. takes moderate (60 min, 3 times a week) execise.
. wears protective padding if she takes part in any heavy contact sports or activities, it is advised however not to partake in such physical sports with a single kidney.
. low salt intake.
. low protein intake.
. no processed foods (microwave meals, takeaways etc.) contains too much salt.
. has a low alcohol intake, doesn't want to make a habit of getting drunk, try changing her drinking habits to a more continental-attitude towards alcohol; social occations etc.

and, finally although easy to say i would ask that she not get to worried, anxious or distressed about her condition... she can live a perfectly healthy life with one kidney and plenty do without realising it!!! it's just that with this diagnosis she now has more of an insentive to do what we all should be doing regardless; which is to eat healthy, not to put on too much weight, drink water and have regualr exercise! hope that helps mate