Is thinking about marrying the sister's deceased husband, with the aim of caring for and protecting her children, despite the opposition of one of the brothers, considered a religious or social error?

May you be rewarded for taking care of your sister's children. If the man is pleasing in his religion and character, then you are not wrong in agreeing to marry him. We advise you to supplicate, perform Istikhara (prayer for guidance), and try to persuade your elder brother to agree to the marriage. If he persists in his refusal, then someone else among your brothers may marry you off. Do not hesitate to proceed with the matter of marriage.