Yes of course.
The seller may or may not agree. If she does, she will ask you to return the garment. Use a Delivery Confirmed service like ExpressPost International.
However, this is very expensive for overseas, and quite expensive even within North America. You may decide that it is too much, since the seller is unlikely to refund the return shipping.
If you return the garment and the seller sends a different garment, you may still have sizing problems. And the seller is not likely to send the replacement free of charge. She may want you to pay for the shipping on the new garment.
This means that getting a replacement costs you $50 or more. Is it worthwhile?
Or you could ask the seller for a refund on return. Depending on whether you are shipping to Canada, USA or overseas, you are still going to be out the cost of ExpressPost shipping. Is it worthwhile?
Perhaps your sister-in-law has a birthday coming up? 
It is important, if the seller does not give exact measurements in the description, that the customer ask. Personally, I have clothing that fits nicely even though the manufacturers say they are sizes from 18 to 24. And you really have to be careful with Chinese made clothing, since they really can't believe how big we Canadians get. Indian seems to be okay for the most part though.
