With the US Embassy, you never know. I think it'd help if your wife's sister is married although it doesn't automatically mean that, if she's not married or divorsed, she'd be denied a visa. She has a family back home, a property, a job... let her HR write a letter stating her salary, for how long she's been working with the company, etc. I'd tell you this - my Ukrainian girlfriend (30, divorsed, no children, no property) came to visit her boyfriend in the US two months ago. He sent her an invitation letter (not stating that he's her boyfriend, of course) and she had no problems at all getting the visa. On a top of this letter of invitation she only had a letter from work stating that she had vacation Jan 10-27 and needed to be back at work Jan28. (May be the fact that she was in the US twice before on student visa and always returned back to Ukraine in time helped, too....)
In any case, you definitely do it; it's worth to try. You write her a good invitation letter that she never had a chance to be with her sister/your wife at the wedding and that it's been two years since they saw each other, and that you'd travelling around showing her the US, etc. I know that Americans are very sentimental about the weddings, it's very important for them, so I heard that it's been working pretty good for the Embassy in Kiev. I'll also have to go through all this in April-May; I want my both parents and my sister and my brother in law to come in visit us in August. I'm sure, that will be an advanture for all of us

! Good luck

!!
Masha