Just a note that if your sister-in-law is gluten intolerant, and not actually coeliac, then it won't be the end of the earth if she accidentally eats a little wheat/barley. She may feel some discomfort (bloating) and the, ahem, after-effects may not be pleasant for those around her, but you won't kill her. But please do check that she isn't coeliac.

swisspea's advice, above, is excellent!