Can my sister claim a medical reimbursement for a pre-existing ailment?

Please understand that an insurance policy covers your medical risk. It means it covers an event, which may or may not occur in future. As in your sister case the ailment exists prior to the day when she had taken the policy, this will be excluded as pre existing ailments (ailment which exist at the time when policy is taken). All pre existing ailments are not covered for a specific period as defined in that Mediclaim policy.