Well, has she had it investigated? If so, what does her OS have to say about it?
A 'large bump' seems to be saying haematoma to me.
As for the leg length, who told her it was 2" different? If it was the surgeon or the PT, it would be worrysome as that's an awful big differential.
But the thing is, there are two 'differentials' which is apparent and actual. Apparent is when you stand with your hips (pelvis) tilted. When someone has a long standing deformity in the hips, the body compensates by tilting the pelvis so the legs seem to be the same length. Then, when the bad hip has a new joint put in, the normal length reasserts itself but the body still wants to tilt the pelvis so it seems for a long time that the legs are a different length when they're really not. It's just her body hasn't yet learned to walk with the pelvis straight again.
The actual length is when a doctor or PT measures from a bony point on the pelvis to the ankle bone on both legs. If that's different then the leg differential is a true one and not 'apparent'.
So has your sister been properly measured (and by whom) and told it was an actual differential?
Finally, you say she's in tremendous pain - what pain killers is she taking, what dose and how often? Does the doctor who's prescribing them know all the facts?