Why are you looking for an outfit rather than your sister? Since it seems she doesn't have to match anyone else, wouldn't everyone be happier if you just said to her, "This is what I'm wearing; could you find something nice in the same ball-park formality level-wise? I kind of had this sort of thing in mind (show Nordstrom's outfit)" and let her pick out her own clothes? Personally in your sister's place I'd be slightly put off if someone else, even my sister, handed me an outfit and said, "Here, wear this," which is effectively what you would be saying.
Second question, why black? Is is because you have a theme going which makes black the colour to choose, or because you think it is flattering to the larger figure? As a generously proportioned woman myself I can tell you that black makes some women look like hags, and picking a colour that goes with your skin and hair type is a better plan. Design is more of an issue than colour as far as figure flattering goes.
I do like the outfit you found; pity you can't find the top as well. But I personally think your sister would be happier if you let her find something that looks good on *her*. It's very hard to successfully buy clothes for other people.