Why doesn’t law protect a caring daughter-in-law’s future rights?

If a daughter-in-law has no right to claim her in-laws' property, despite staying with them for decades, why did she take care of them if they are not protecting her future? What is this double standard in law? If a daughter-in-law doesn't care for her in-laws, the law considers it wrong behavior, but if a woman spends her time caring for them, why don't the in-laws protect her future? Is the law biased against women in this regard?