The Immaculate Conception refers to Mary’s own conception. She is the only human conceived without sin (except Jesus). She was conceived without sin in order to be the vessel, the Ak of the Covenant, the one who carried Jesus himself.
She was a virgin when she conceived Jesus in her womb. She became pregnant only by the Holy Spirit, not Joseph or any human being. She remained a virgin all of her life. Jesus’ brothers and sisters mentioned in the bible is a mistranslation as I understand it. In the original text a word was used which referred to Jesus’ kin. In those days, it was customary to refer to cousins as kin, i.e., the translation to brothers and sisters. There are apologetics reasonings which support the ever-virgin concept beginnning with the oral tradition of the early Christians. The Immaculate Conception concept was a declared teaching by a Pope in the early or mid 20th centruy but it had been accepted unofficially I believe more or less throughout church history.