"(W)as/is a perpetual virgin?" You got me on that one. Sorta like being a little bit pregnant.
The Holy Scriptures refer to brothers and sisters of the Lord, but the Church (both in the East and West) has always taught that they were family relations of the Lord, i.e. cousins or "step" brothers and sisters. The word translated has this broad meaning, in Hebrew, Aramaic, and Greek. There have been several Posts/Articles on FR concerning this topic, and I off hand don't know where to direct you. Maybe somebody a little more up on where to go, can.
In any event, I thought it would be fun to mention a treatise that St. Jerome wrote in the 4th Century concerning this very topic. I had always remembered the treatise as being very good and "current". I checked the internet and voila! Here it is:
Not too long, and it covers Catholic thought and teaching on this subject very well from one of the great Fathers of the Church. He says it better than I ever could.
but knew her not until she had borne a son; and he called his name Jesus.
I would think the phrase "until she had borne a son" is indicative of Joseph having relations with Mary.