Brothers and Brethren.
There are two accepted forms of the noun: adviser and advisor.The plural forms are advisers and advisors.The plural possessive forms are advisers' and advisors'.
Plural of abacus: abaci or abacuses.
There are two accepted plural forms of the noun vertex: vertices and vertexes.The plural possessive forms are vertices' and vertexes'.
Octopi and octopuses are both accepted plural forms for the singular octopus.
There are two accepted forms of the noun: adviser and advisor.The plural forms are advisers and advisors.
The plural form of the noun brother is brothers.The plural possessive form is brothers'."Mercy secretly studied with her two brothers'tutors."
There are two accepted plural forms for the noun cargo, they are cargoes and cargos.The plural possessive forms are cargoes' and cargos'.
There are two accepted plural forms of the noun pharynx:pharyngespharynxes
Brothers is the plural form of brother.
Isthmi is plural. There are actually two plural forms. I will list them from singular to plural. Isthmus - Singular Isthmi - Plural Isthmuses - Plural
There are two accepted forms for the plural of 'tuxedo', tuxedos or tuxedoes.
The two accepted plural forms of the noun 'fucus' are fuci or fucuses.