Failing to do one's duty is an unjust. That is why, parents fail to take care of kids as well as kids fail to take care of parents , are acts of unjust. Unjust does not necessarily mean serious crimes only. Neglience of duty is also in a sense, crime and thus an act of unjust.

Many think these are optional. But, the truth is these are not optional duties. Always perform your duty. Duty is not limited to towards parents and kids.