it really doesn't matter- even if there is a slight difference- you can use which ever side you like best---sometimes one side will be a little more vibrant than the other- but batiks are not printed fabrics- they are dyed-so both sides are beautiful and very close to the same-
i tend to use which ever side i want/like with every fabric- i pay for both sides-no reason not to use both if the back side works better for my project than the (right side) i use it- sometimes that muted/washed out color is just right. but true batiks like hand dyed fabrics can be used with no noticable difference....
tell your sister---there is no (Right or Wrong) only different-