As a married woman with a child, if you are honest with yourself and use common sense, you will not get sexually involved with your brother-in-law (or anyone else for that matter). A marriage is a challenge enough without adding more problems to it by ill-advised behavior.
This is not incest. Incest is when blood relatives have sexual relations. You and your brother in law are not blood related. Imo, that's a question only you and your husband can answer together. If you are both open to an open marriage and both agree to it then that's what counts. I'm not saying it won't get weird or anything but really that's a question that only you two can answer. Everyone's marriages are different and some do prefer open marriages. However, if both do not agree to it then it's cheating and could ruin your marriage. You have to be ready for any kind of downfall in the future from it too should anything go wrong. Good luck.
Nothing wrong going on here. Live your life and have fun. Do not let others tell you this is wrong, it is in no way wrong, bad, gross, abnormal or anything else. It sounds like you, your hubby and you brother in law all have a great life.
This is an old post, but if the o.p. is still checking in, I would say something in between YardBull's comment and DallasMale's comment. It is very complicated to have a relationship with one person while being married (i.e. a committed relationship) to another. It's even harder when a child is involved (kids see everything, my dear). If you can manage the difficulties and the complexities, I am not judging a consensual relationship where the husband knows also that it is going on. I do ask why your husband is not bothered by what his brother and you are doing -- sibling rivalry can be very painful. Is he wanting out? But if he is genuinely not bothered, and if you are managing the difficulties and complexities, and if you are super aware of the model you are presenting to your child and doing a great job of hiding it, from the child and everyone else in the world, then as Dallas says, live your life and have fun. I just think you are taking on a great deal in trying to make this work.