The chances of these people changing their behavior through counseling and meditation are very good. But will they cooperate to come to the psychiatrist? That is the main problem. Now that your sister is at your home, the strategy you can follow is to make the elders negotiate a deal with your brother in law and his family. They should tell them politely that all three ie., your sister, her husband and mother in law should visit a psychiatrist and undergo counseling, and that you would send back your sister to her husband only after the psychiatrist gives the green signal. Until then you should put your foot down and not budge. Nobody has the right to beat his wife even if she has done anything wrong(in his view). Your elders can also tell them subtly that if the problem is not solved amicably you would be obliged to lodge an FIR against them.

Definitely  counselling would be a better option and solution for there problems. And I  would suggest  that both of them attend the sessions so that the counsellor would be able to understand  both of them and would be able to guide them as appropriately.

Both mother and husband seems to suffering from psychotic disorder, they need counselling and medications but they will not agree for treatment, you will have to put your foot down and convince them to take treatment