Must the sister pay her siblings for purchasing their deceased mother's apartment, knowing that the apartment will be returned to the owner and none of the heirs will buy it?

There is no objection to purchasing the apartment from its owner with his consent and without coercion. The heirs have no right to the apartment itself when it is purchased from its owner. Rather, they have the right to benefit from the leased property if the lease contract remains valid, because a lease contract does not terminate upon death, unless the contract was void, like the old lease contract which is فسخ (annulled/dissolved) due to the tenant's insistence on occupying the property without the owner's consent and for a meager rent.