The allowance number on Form W-4 decides how much of federal and state income taxes will be withheld from your periodic payroll income. The tax amount withheld and remitted to taxing authorities will be reconciled during your annual income tax return preparation. Depending on the amounts, you could owe or be allowed for a refund.
It appears that your sister and mother can be claimed as "qualified relative" dependents. Therefore you can claim them on your W-4.
Thank you.