Learn Dutch: De broers, wier erfenis groot is, willen de baas opvolgen. - The brothers, whose inheritance is large, want to succeed the boss.

The relative pronoun refers back to de broers (the brothers), which is plural. In formal Dutch, you use wier for feminine and plural nouns, and wiens for masculine and neuter singular nouns.

Would I use wier like this in everyday conversation?

Usually, no. Both wier and wiens are considered quite formal and are mostly found in written Dutch or formal speeches. In everyday conversation, a Dutch speaker would typically use van wie instead, making the clause: , van wie de erfenis groot is,.

Why is the verb is at the end of the middle clause (wier erfenis groot is)?

Because it is a relative clause, which functions as a subordinate clause. In Dutch, subordinate clauses always push their conjugated verbs to the very end of that specific clause.

Opvolgen is a separable verb. Why isn't it split up here?

Because the main clause uses the modal verb willen (want). Modal verbs push the main verb to the end of the sentence as a full infinitive. When a separable verb sits at the end of the sentence as an infinitive, its prefix (op-) remains attached.