Short answer. A promise from you is not enough. Article 750 requires your mother to actually reserve, in full ownership or usufruct, sufficient means to support herself and anyone by law entitled to support from her. Without that reservation, the donation can be reduced on the petition of any person affected.
What the law says
The donation may comprehend all the present property of the donor, or part thereof, provided he reserves, in full ownership or in usufruct, sufficient means for the support of himself, and of all relatives who, at the time of the acceptance of the donation, are by law entitled to be supported by the donor.
Civil Code, Article 750 — Giving Away Everything You Own. Read the full provision →
What the law says
Without such reservation, the donation shall be reduced in petition of any person affected.
Civil Code, Article 750 — Giving Away Everything You Own. Read the full provision →
Donating everything is allowed, but on one condition
Article 750 does not stop your mother from donating all of her present property to her children. It states: the donation may comprehend all the present property of the donor, or part thereof, provided he reserves, in full ownership or in usufruct, sufficient means for the support of himself, and of all relatives who, at the time of the acceptance of the donation, are by law entitled to be supported by the donor. The word "provided" is doing the real work here. Giving away everything is conditioned on keeping enough, in ownership or at least in usufruct, to actually support herself and her legally entitled dependents.
A promise from you does not satisfy the condition
The article speaks of the donor reserving means of support for herself, not of the donees promising to provide it later. Your commitment to support her is a personal arrangement between you and her; it is not the reservation of property or usufruct that Article 750 actually requires. If she donates everything and relies only on your promise, she has not met the condition the article sets, regardless of how sincerely that promise is made or how reliable you intend to be about keeping it.
What happens without a proper reservation
The consequence of skipping the reservation is specific: without such reservation, the donation shall be reduced in petition of any person affected. The donation is not automatically void from the start, but it becomes vulnerable to reduction if someone affected, which could include your mother herself, or another relative she is legally bound to support, brings a petition. The excess that leaves her without sufficient means for her own support and that of her dependents can then be cut back.
What this means for structuring the donation
If your mother wants to donate her assets to you and your siblings, the safer path under this article is to have her formally reserve sufficient property or usufruct for her own support and for anyone by law entitled to support from her, rather than relying on an informal understanding that the children will take care of her. That reservation, not a promise from the donees, is what protects the donation from being later reduced on the ground that it left her without adequate means.