An infatuation is a driving force that makes people break up and realize big changes in their life. An infatuation could be the beginning of a life-long relationship, but it can also be a temporary crush. It is too early to determine which of these two cases your situation is.
People with an infatuation can become as ruthless as drug addicts. Children and partners are sacrificed. The least ruthless you can do is to find your own place to live. Your husband would suffer even more if you moved directly into his brother's apartment. One of my patients had a similar problem and she was dismayed at the fact that many took her husband's side. She lost most of her friends and that was a consequence she had not have thought about.
It can happen that people become infatuated when they have a problem they cannot solve. Some escape their daily life by starting a new relationship and concentrating on it, instead of solving their own problems. A crush gives an inebriation very similar to the one caused by drugs. Life becomes sweeter and the crush or the drug is the most important thing in one's life. Other people may suffer. ( In your case, you write that your husband is aware that you do not love him anymore, so it is difficult to say how much he will be hurt if you leave him. Perhaps he is already preparing for this, and has his own plans for the future). One never knows, but the consequences of this inebriation may catch up with one later in life.
It happens that women leave their families for a strong infatuation, but after a while they return to their husbands again. The grass was not so green on the other side. Only the future can tell how your situation will develop.
You have had your process going on for 6 months and now you have decided how you want to solve this. Your husband has not had the chance to meet someone else. He needs to go through his own process, based on despair, anger and eventually reflect on his alternatives. Maybe he will fall in love with your sister or your best friend.