My Dear,
I don’t think you are being fair to your husband. The fact that he is comfortable around your sister might stem from the fact that he loves you and regards your sister as his sister too. This is quite normal among in-laws. I don’t think you should read a different meaning into their relationship.
Your sister being attractive does not make her stupid. But if your suspicion is growing stronger, it’s best to have a chat with your sister and make her know that she should never do anything that will hurt you or bring shame to your family.
But, you have to be careful how you approach her with the issue. If she’s not interested in your husband, it might backfire because you’ll be planting the idea in her mind. So I suggest that you guise your suspicious and present the issue to her in general terms. Her reaction can help you discern the true position between them so you can decide how best to handle the situation if there is anything unseemly.
It is not a crime to admire attractive people; most people are guilty of this without having any ulterior motives. Make sure you are paying attention to your self-care, grooming yourself and looking good. A good number of women are guilty of letting themselves go once they become wives and mothers. Taking care of yourself will help you to be self-assured about your relationship with your husband because it will keep his focus on you. Good Luck.