You have your hands full with this one! You don't need to deny your own natural bisexual urges, but you don't need to act on these urges either.
We humans have all kinds of urges and impulses, sexual, bisexual, incestuous, murderous, and so on. In many cases, acting on our urges is purely self-gratifying (it feels good in the moment), but ultimately damaging in the long-run. If you are furious at your boss and act on your rageful impulses by telling him/her off, you are out of job. That's a good example of how acting out a feeling can get you into trouble.
The key is to use to choose when to act and when not to. Sleeping with your mother-in-law can only bring disaster. The fact that she is hitting on you feels very incestuous to me. Sleeping with you is practically like sleeping with her own daughter. You have to really think through what would be the consequences to you, your husband, and your marriage if you were to act. Next you need to seriously consider whether it's wise for you to act on your bisexual urges with anyone else.
You are playing with fire here and you need to be careful not to get burned. How would your husband feel about your sleeping with another woman? What could be the risks and the benefits of doing so? Many people choose to leave such urges at the level of fantasy, while masturbating or making love with their partners. They also may gratify these urges vicariously by watching pornographic videos in which bisexual acts are performed. Some couples even verbalize these urges to each other while making love.
You have a lot of thinking to do. Meanwhile, keep your panties on.