The Immaculate Conception refers to Mary's conception without original sin, not to the conception of Jesus as many mistakenly believe. So, remained a virgin until at least after the birth of Jesus, which is referred to as the Virgin Birth. I don't believe we know whether she remained a virgin after that.
He had siblings. The Greek "adelphos" is unambiguous, it is quite definitely "brother" and not "kinsman". One was an apostle (James).
Joseph had seen his beloved give birth to a child that wasn't his. He'd have impregnated Mary as soon as humanly possible after that, to prove his masculinity if nothing else. The idea of "ever virgin" could only have been conceived (pun intended) by a celibate priest, married couples don't behave like that. Sex is one of the few compensations for the fact that raising a baby is one hell of a lot of work.