This is a wild & inhuman idea, certainly. Ask your wife about this first in privacy to see her reaction. Don't be so much childish & extremely selfish. If you ask your sister in law, she may or may not allow you to do that. Most likely, she will assume you a great cheater. If she allows & you do that out of innocence, one day she is sure to get pregnant. Even before getting pregnant, it will invite so many problems for you, your wife & the whole families of both sides, definitely ruining your marriage. Have sex with your wife as much as you can. That is the best solution. Your sister in law may be very sexy by look but not efficient enough for enjoying sex physically & emotionally. Both are totally different things. After having sex with a virgin girl first, the girl herself starts coming to the man again & again secretly to enjoy but there are some natural characteristic changes in her voice, way of talking, facial expression, routine & behaviour. She will sleep longer & longer & many times sit alone thinking deeply for long time, come in notice of others. Parents understand this change but things are already out of hands. Please don't think of spoiling that innocent sister in law & behave responsibly like a God fearing highly principled married man of high moral standards. God always punishes culprits, you can't imagine. Please take every care & never think of doing any such thing. Ask your wife to make herself more attractive & lovable than she is now.