Short answer. One-half of your mother's estate is legally reserved as the legitime of her parents. Under the Civil Code, the legitime of legitimate parents or ascendants is one-half of the hereditary estate of their children or descendants when no children or descendants of the deceased survive them.
What the law says
The legitime of legitimate parents or ascendants consists of one-half of the hereditary estates of their children and descendants.
Civil Code, Article 889 — Legitime of Legitimate Parents. Read the full provision →
What is a legitime and who has one
The Civil Code reserves a portion of every person's estate for compulsory heirs — people who cannot legally be cut out of the inheritance. These reserved shares are called the legitime. Legitimate parents and ascendants are compulsory heirs of their children and descendants. This means that if your mother left a will that gave everything to charity or to friends, that will cannot consume the half that belongs to her parents as their legitime. Any disposition that impairs the legitime can be reduced.
The one-half share in concrete terms
Article 889 of the Civil Code is direct: the legitime of legitimate parents or ascendants is one-half of the hereditary estate. The other half — called the free portion — your mother could dispose of as she chose, subject to any rights of illegitimate children or a surviving spouse. In your situation, since she left no children, her parents inherit as compulsory heirs and their combined legitime is one-half of whatever she left. They would divide that half between themselves if both grandparents survive.
The surviving spouse and illegitimate children
Article 889 itself notes that the free portion is "subject to the rights of illegitimate children and of the surviving spouse." This is a reminder that the calculation does not end with identifying the ascendants' share. If your mother had a surviving husband, his share must also be computed. If she had illegitimate children, their legitime is likewise carved out. The final distribution depends on who the compulsory heirs are, and those with higher priority in the succession order affect what the ascendants ultimately receive. A lawyer can run through the full picture once all the heirs are identified.
When this rule applies and when it does not
The parents' one-half legitime under Article 889 applies when the deceased left no legitimate children or descendants. If your mother had had children — even only one surviving child — the picture changes completely, because legitimate children are a higher class of compulsory heir and they would exclude the ascendants from inheriting as compulsory heirs. The ascendants' claim only becomes operative in the absence of children and descendants. It is also worth noting that legitimes attach to the net hereditary estate: debts, expenses of last illness, funeral costs, and valid charges on the estate are settled first before the remaining estate is divided.