No, it is not a sin. Without prayer the souls dies. Prayer necessitates grace.
Romans 12:12 Rejoicing in hope. Patient in tribulation. Instant in prayer.
Ephesians 6:18 By all prayer and supplication praying at all times in the spirit;
I recommend saying the Rosary everyday, all 15 decades each day.
St. Louis De Montfort (+1710): “Blessed Alan de la Roche who was so deeply devoted to the Blessed Virgin had many revelations from her and we know that he confirmed the truth of these revelations by a solemn oath. Three of them stand out with special emphasis: the first, that if people fail to say the Hail Mary (the Angelic Salutation which has saved the world) out of carelessness, or because they are lukewarm, or because they hate it, this is a sign that they will probably and indeed shortly be condemned to eternal punishment.” (Secret of the Rosary, p. 45)
St. Louis De Montfort (+1710): “….there are some very sanctifying interior practices for those whom the Holy Ghost calls to high perfection. These may be expressed in four words: to do all things by Mary, with Mary, in Mary and for Mary; so that we may do them all the more perfectly by Jesus, with Jesus, in Jesus and for Jesus.” (True Devotion to Mary #257)
St. Louis De Montfort, True Devotion to Mary #39: “We must not confuse devotion to the Blessed Virgin with devotions to the other saints, as if devotion to her were not far more necessary than devotion to them, and as if devotion to her were a matter of supererogation [extra special devotion].”
St. Louis De Montfort (+ c. 1710): “By this practice [the True Devotion to Mary which he teaches], faithfully observed, you will give Jesus more glory in a month than by any other practice, however difficult, in many years…” (True Devotion to Mary #222)
St. Louis De Montfort: “… many others have proved invincibly, from the sentiments of the Fathers (among others, St. Augustine, St. Ephrem, St. Cyril of Jerusalem, St. Germanus, St. John Damascene, St. Anselm, St. Bernard, St. Bernardine, St. Thomas and St. Bonaventure), that devotion to Mary is necessary to salvation, and that… it is an infallible mark of reprobation to have no esteem and love for the holy Virgin.” (True Devotion to Mary # 40)
Regarding the Holy Rosary, Sister Lucia told Father Fuentes in a famous 1957 interview:
“Look, Father, the Most Holy Virgin in these last times in which we live has given a new efficacy to the recitation of the Holy Rosary. She has given this efficacy to such an extent that there is no problem, no matter how difficult it is, whether temporal or above all, spiritual, in the personal life of each one of us, of our families, of the families of the world, or of the religious communities, or even of the life of peoples and nations that cannot be solved by the Rosary. There is no problem I tell you, no matter how difficult it is, that we cannot resolve by the prayer of the Holy Rosary. With the Holy Rosary, we will save ourselves. We will sanctify ourselves. We will console Our Lord and obtain the salvation of many souls.”