Wow Margie your gem of a sister in law deserves a big thank you for all of that hard work! I totally understand why you want to recognise her efforts with something aq little special.
Gift certificates are generally a very safe bet - and so easy to pop in the mail too! If your sister-in-law would like a little pampering after her dieting efforts - I recommend a thank you gift certificate which she can use at a local spa - or one from an online beauty product specialist company. I like FeelUnique.com because they have all the top brands, they deliver internationally and they have GREAT discounts!!
Other nice thank you gift certificate ideas could be travel or gardening related - hope this helps and your sister-in-law is delighted with whatever surprise you send her way!