My mother has passed away. We found a video in which she stated that she is leaving all her belongings to my brother's children. Is this will valid ?

No. For a will to be valid, it must take one of the forms provided for by Article 969 of the Civil Code: holographic, authentic, or mystic, all of which require a written document. A will made on video is therefore null and void and cannot produce any effects (Reply Toubon: AN 1-12-1986 n°11866).

Note : Even if the will is not valid, the heirs can agree to execute it (Civ. 1e, June 22, 2004, n°01-14.031).