Thank you for clarifying. So they aren't being evicted, but the landlord has merely decided not to rent to them anymore. Unfortunately, so long as the landlord gives proper notice (i.e., at least 30 days), then it is lawful for the landlord to terminate the tenancy for the purpose of letting the landlord's children live in the house. After all, your brother and his wife could have also terminated the tenancy with 30 days notice, in which case it would have been the landlord who needs to make alternative plans.
In any event, if I were your brother, I would simply ask the landlord for a little more time if necessary. For example, if he needs to wait until January, then he should ask the landlord. Since he's been living there for 20 years, I assume he has a good relationship with the landlord, and so the landlord may be accommodating. If push comes to shove, and the landlord refuses to give your brother any more time, then your brother could simply refuse to leave for a short period, in which case the landlord would be forced to file for an eviction. Although the court would almost certainly side with the landlord, it would probably be a few weeks before it gets to court, particularly because of the holidays. By then, perhaps your brother will have secured new living accommodations, and the eviction will be moot.
The most important thing to take away from this, however, is that there is no realistic way to force the landlord to continue to rent to your brother and his wife. The landlord has the right to terminate the tenancy with notice (as mentioned above). So, whether your brother finds a new house by December or January, it'll have to happen at some point in the near future. There is no way to get around that fact, and if your brother waits until he is evicted, it may buy him a few extra weeks, but the last thing he'd want is to be forcibly removed by the sheriff's department. Neither your brother, nor the landlord, nor the sheriff's department would want that. So, he does need to be actively looking for new living arrangements.
Does that answer your question? Please let me know if you need clarification, as I am happy to continue helping you until you are satisfied.