Having children with someone is the best reason to work harder on your relationship that I know of. Some might say that marriage is the best reason, but that really depends on how much the institution of marriage means to you.
Of one thing we can be sure: marriage as an institution doesn’t mean much to kids. Long gone are the days where there was any kind of credible argument that your parents not being married causes you harm. We now know that what does matter is the quality of the relationship with the consistent loving adults in their life, regardless of their relationship’s contractual status.
We also know that the impact of conflict, before and after separation, is what harms children; not the act of separating itself. I would go further and suggest that not just obvious yelling, arguing and conflict, but also coldness, distance and a relationship that no longer looks or feels loving also has a negative impact.
Our children live in the emotional atmosphere we create for them, and at an emotional, sometimes even non-verbal level, they learn from that. We might convince ourselves staying in a relationship that is no longer working means at least the family stays together, but we must also be mindful of what we teach our children by doing so. What we show them about how relationships should look and feel.
Without knowing more about your brother’s relationship it’s hard to say which way he likely needs to go, but my challenge to him would be to use his love for his children to either work harder and more actively on his relationship by approaching the difficult conversations - via relationship counselling if need be, or if things truly are beyond rescue, find the support to move on and establish a peaceful and amicable co-parenting relationship.
It’s hard to escape the fear that separating harms children, and it’s not entirely untrue. But being around conflict, angry, bitter, or cold, cut-off adults harms them more. Having two families, two houses and two different places to call home can also be very disruptive.
But at least it allows for happy adults, which leads to happy children - and a model of life that values emotional health as much as attachment to old ideas about forever after.