Here's a good chart. If you subtract the suggested quilt size from the mattress size, the drop is about 7". This is about the length I usually use.
http://fgqg.com/Quiltsizes.htm
Unless it's a surprize, you might want to get the mattress dimensions from your cousin, see if she'd like a longer drop, more like a spread. Also, there are the extended twin mattresses that are a bit longer.