The word "heos" in Greek doesn't imply a reversal in state after the specified event like it does in English though. In the Bible it often only indicates what happened up until a certain event, with no implication about what follows on after. So, for example
"And Saul's daughter Michal bore no children from that day on until the day she died." 2 Samuel 6:23 - that obviously doesn't imply she had children after she died 😁
"The LORD says to my Lord: “Sit at my right hand, until I make your enemies your footstool.”" Psalm 110:1 - that doesn't imply that Christ will stop sitting at God's right hand after.
Also, with regards to "brothers and sisters", if you've ever studied a Kinship module in Anthropology, you learn that different cultures have different systems for describing kinship relations. For example, the word for your mother's sister's children might be the same one you use for what we call siblings, but you might refer to your mother's brother's children as your cousins. So Jesus' "brothers and sisters" may well be cousins, or children from Joseph's earlier marriage, as Joseph was an elder betrothed to her to be her guardian and protect her virginity.
There's also theological reasons why Mary was a perpetual virgin, that she is the Ark of the New Covenant, giving birth to God the word who saves and justifies, ie holy, set apart and untouchable. She had total consecration to God as her lifelong vocation in her role as Theotokos - God bearer, as described by St Athanasius and St Jerome.