Will the questioner be held accountable for leaving his sisters without hijab and his brother without prayer, knowing that their father has passed away?

It is obligatory to advise the brethren, remind them, and condemn their apparent transgressions, in accordance with the Hadith: "Whoever among you sees an evil, let him change it with his hand; if he is unable to do so, then with his tongue; and if he is unable to do so, then with his heart—and that is the weakest of faith." So, if one enjoins good and forbids evil to the best of his ability, their non-response will not harm him, as Allah Almighty says: "O you who have believed, عليكم أنفسكم la يضركم من ضل إذا اهتديتم" (Surat Al-Ma'idah: 105).