Immigration sponsorship question

My mom is a legal resident, and anticipating in becoming a US citizen next year.

Once she becomes a US citizen, can she sponsor my father (who entered without inspection)?

Can she sponsor my sister and I as well? (we are both over 21 yrs old).

It will be easier to sponsor your father. I don't think the over 21 category is considered an immediate relative. She can sponsor all of you, but a waiver will be required for each and every applicant. It would be best if she sponsored your father only and use the waiver process while in the U.S.