Is it permissible for brothers living in one house to share in the Eid sacrifice (udhiyah), and for whom will the reward be?

It is permissible to share in the Udhiyah (sacrificial animal), even if it is a single sheep, if the participant is a relative of the one who included him in the sacrifice, resides with him, and he provides for him (even if voluntarily). If any of these three conditions (kinship, cohabitation, and provision) is not met, then sharing is impermissible. So, if the owner of the Udhiyah is the one providing for them, it is permissible for him to include his siblings with him in his sacrifice, and it will be sufficient for all of them. They will all receive the reward of the Udhiyah, and the owner of the Udhiyah will receive a greater reward because he provided the expense.