Is it permissible to boycott younger sisters who do not pray or obey, especially when the mother supports their actions?

One should not despair of guiding one's sisters and of their performance of prayer, but rather continue advising them, instilling fear of Allah in them, and encouraging them with what He has prepared for the obedient, as in His Almighty saying: "Indeed, Muslim men and Muslim women, believing men and believing women, obedient men and obedient women, truthful men and truthful women, patient men and patient women, humble men and humble women, charitable men and charitable women, fasting men and fasting women, men who guard their private parts and women who do so, and men who remember Allah often and women who do so – for them Allah has prepared forgiveness and a great reward." (Al-Ahzab: 35). The mother should be advised gently and kindly to be an aid in the sisters' righteousness.