Isaiah 7:14 calls mary a "Almah"
Isaiah didn't talk about Mary or Jesus but talked about his own time. Messiah is this context meant savior in the political sense. It's not a prophecy for the distant future, only in the Septuagint much later. But don't take it from me, take it from Dan McClellan.
Jesus was the first born. Even if Mary was a virgin when he was born, this isn't necessarily the case for his younger siblings