After Jesus was born Mary and Joseph were just like any other married couple, they had sex, they had children.
No they didn’t. Jesus is an only child. Where are you getting these ideas?
The quotes in the Bible about brothers and sisters refer to brethren or relatives.
But as TC just affirmed, it does not matter if Scripture describes marriage as "cleaving," and sanctions abstinence just for a short time in marriage, and other things mentioned above, and manifests extraordinary aspects of its subjects (from having more than 10 fingers to sinlessness) but says nothing about Mary being sinless and a perpetual virgin; if Rome sees nothing contrary to it, then since she has declared herself infallible, therefore RCs must believe it, as if it were from God Himself, and are not to seek to verify it by Scripture.
With that, they can forget about converting those who seek to be like the noble Bereans.