For $330 (assuming you get the rebate), particularly when you consider the cost of a Windows license, I think this is a very fair price for what you're getting.
That said, it does have some weaknesses that may need to be addressed.
1. The PSU is an old, obsolete design, probably overrated; likely suitable for the machine as-delivered, but unlikely to support even modest expansion.
2. The CPU is a low-power "T" model, which means that it will be a few tiers weaker than its i3 designation suggests. For the stated purposes however, I suspect it will be adequate. I'd expect it might struggle in large WoW raids, so if that is a concern, pass on it, otherwise it's probably okay.
3. The RAM is probably "enough," but there is no indication as to how it is arranged. Is it 2x2GB (faster initially, but an upgrade means tossing it), or 1x4GB (slower initially, but an upgrade means just adding another similar stick) ?
4. To play games, it WILL need a graphics card added. Depending on the size of their monitor, a HD7770 would be a decent choice, but will also need a PSU upgrade. If their monitor is only 1368x768, a HD7750 might be enough, and should run on the included PSU (even if it's only good for 250W).
You know your sister's needs. If she's only a casual gamer and doesn't need high performance, this system plus a graphics card may be suitable. It does seem to be a rare prebuilt that, if performance is sufficient, is worth the price being charged. Just make sure the CPU will be enough, particularly for the games she and her husband play. A CPU upgrade is likely possible, but at that point it will cost enough that it makes more sense to build a rig for them.