I swear it seems to me I've seen this exact item discussed here before but I can't find it!
However, looking at the picture you posted and going to the Marta Locklear Photography website and checking out other pictures she has posted with that wrap it looks like a tube knit in stockinette with large needles. She increases it as the tube moves upward and opens it up 2/3 of the way up with a button closure. Probably a m1 increase on either side of a row about every other row or every few rows so as not to create holes. I am by no means a knitting expert and have not been knitting long enough to say all this with complete certainty, but if your sister has been knitting for a while maybe she can improvise with that information.
Does what I say make any sense to anyone else? Its just my thought upon seeing it
Sorry I couldn't be more helpful though. I'll keep searching