Here is how inheritance rules are simplified: “Love descends, then ascends, and then spreads.”
On one hand, “descends” means that the children inherit first and, in case of predecease, the children’s children succeed. The surviving spouse inherits like a child; hence, he/she gets a share equal to a child’s share.
“Ascends” in turn pertains to the deceased person’s parents who inherit in the absence of descendants and spouse.
On the other hand, “spreads” refers to collateral relatives, who are the deceased person’s brothers and sisters, or nephews and nieces who inherit in the event the other heirs in the order of succession are no longer alive.
Please note that this principle applies only when there is no last will and testament because, should there be one, half of the estate, termed as “legitime”, of the would-be deceased person is reserved by law to his/her children and surviving spouse while the remaining half or the “free portion” can be bequeathed even to a complete stranger to the family.
Kindly bear in mind further that the same simplified rule finds relevance only to legitimate relationships since illegitimate children only gets one-half of the share of a legitimate child. There is also an “iron curtain rule” in that illegitimate children are not entitled to succeed to and inherit from the legitimate relatives of the deceased person.