Her mother telling her this so soon after OP's father died is extremely selfish. She probably felt guilty for years, and took the first opportunity she could to unload this bomb shell on her daughter while she is still trying to cope with the emotions of the death.

If the mother was actually interested in letting the daughter know the truth for her best interest, she either would have done it years ago, or put some time and emotional distance between his death and admitting the truth. Now those two things will be linked in OP's emotional psyche and creating confused feelings.

If OP reads this, what others are saying is correct. The man you raise you is your father. This doesn't change the relationship you had with him, and to him, you were always his daughter. Your siblings are your siblings. Also, moving forward, it should be entirely your decision who knows about your parentage and who doesn't. Don't let your mom out you to your siblings or anyone's else without your express permission.