As you have rightly noticed it is very difficult to stop having sex, especially when it is forbidden sex. The thrill of it rarely stops till you are caught and shamed. That is why, so many people end up having sex with their Ex.
How is the relation with your Husband? Is it strained because he is cheating on you? One way to stop having sex with your Devar, is to pay more attention to sex with your hubby. I agree this may be difficult till such time as he is sharing his cock with some other woman. You will need to communicate with him and gently ask him to stop his relations with that other woman. You can take the help of his parents in the matter.
But, I really don’t know what your response will be if he is aware of your illicit relationship and questions you about it. Don’t be under the impression that he does not know. The only reason he may not be broaching the subject, is because he is enjoying young pussy outside.
Have you stopped to think, what if your devar refuses to stop having sex with you? What if he threatens to tell his brother? You are stepping on dangerous ground, if you give into his wish of having his child. That will complicate matters in the future. Just put your foot down and insist that he use a condom, if he wants to have sex with you.
If you really want to stop having sex with your Devar, you will need your Husband’s support. Get him to ask your Devar to find other accommodation.
Some difficult decisions you have to take. You have taken the first one, by coming to this Site for Advice.
Best of Luck.