My sister-in-law issue is bothering me. Please have your say, girls and boys please!?

It sounds like she wants to know what it is like to sleep with you. This is very common among sisters. It is the sibling rivalry thing I guess. You should press her just a little to see what she wants just to be sure. Then decide what to do-tell your wife or a free one from sis. Sense she is married also you might be able to get away with it with no strings attached.