Is it permissible for students of modest means to share in one Udhiyah (sacrificial animal), or is it permissible for them to sacrifice one Udhiyah on behalf of their parents (two brothers, one of whom is deceased)?

It is permissible to share in the sacrifice (Udhiyah) if it is a camel or a cow (among seven people), but it is not permissible if it is a sheep, because a sheep suffices for one person and his household.

It is not permissible to sacrifice on behalf of a living person except with their permission, unless it is from the perspective of a communal sunnah (Sunnah Kifayah) for the household, or if the sacrificer includes others in the reward of his sacrifice and slaughters for himself.

As for sacrificing on behalf of the deceased, the preponderant opinion is that it benefits them and its reward reaches them. So, if the living father gives permission for a sacrifice to be made on his behalf, and he wishes to include his deceased brother in the reward of the sacrifice, there is no objection.