My mom didnt get a share from her late husbands estate, is this fair and just?

my mom didnt get a share from the estate of her late husband whom the latter inherited from his parents and said person died so many years before his parents passed by too. The contention of his relatives is that my mom though theyre married cant take a part of it because she sired an illegitimate child and thats me. My mother have me when she bacame a widow. Furthermore, said estate is not conjugal considering its an heirloom from his parents so, the sharing would be among my step brothers and sister only. Is this fair and just for my mom?

Its absolutely fair and just. If the estate was meant to be left to your mother it would have stated it was left to her. That it wasn't was a personal choice of the parties involved. If it is an heirloom estate being passed to natural relatives only then neither you nor your mom would qualify anyway. Its solely the decision of whomever wrote the will as to who gets what part of the estate. You and your mother may not like that but thats the way it is.

If her husband died before his parents, he didn't inherit anything from them.

Yes. Also legal. Her former in-laws do not owe her anything.
While it may seem harsh to her (and you) that they refused to treat her as a dear son's widow, the money and property is theirs to do with as they will.

abosultely

Unfortunately, fair and just is not part of the equation. Your late husband died before his parents did. I am sorry for you loss, but there is no law that states his parents have to leave you anything at all. People can pass on their assets to whomever they want to. Heck, people have left million dollar estates to pets.

Inhertance is only assured to a spouse, and even sometimes, that can be passed over, depending on the will.