Siblings or children of people killed in the IDF serve in combat units only with parental consent. You'd think this would be a good thing, enabling bereaved families to avoid losing a second family member. The problem is that in many cases, the young man (it's usually men) insists on serving in a combat unit, and sees his mother (if she's the widow) or his parents (if his brother was killed) as blameworthy for not allowing him to serve as he "ought". Sometimes the parents give in; and sometimes after they do, the young man is killed, leaving the twice bereaved parent with the additional grief of having allowed it to happen.
I don't know why the IDF doesn't simply refuse to recruit bereaved young men to combat units. Back in Israel's early years there may have been a serious dearth of qualified young men to serve in some units, but there's hardly such a problem these days; moreover, there's no lack of important jobs that need to be done which don't involve combat. The poor mother shouldn't be the one who decides, and the one to face the reproach of her young son who doesn't understand mortality, even after having lost a brother or father.
Apparently the army is finally recognizing the problem, though according to Tzachi Hanegbi, they've still not reached the position they should be in.