Was Mary a sinless perpetual virgin? Well, scripture says that the only person that never sinned was Jesus (Rom 3:23, Heb 4:14-15, & 2 Cor 5:21), and it also implies that Mary had children with Joseph after Jesus (Acts 1:14), and it specifies that they had intercourse after Jesus was born (Matt 1:25). This seems like a non-biblical church tradition. Believers are only accountable for believing what is written not what we are told out of tradition. There are a lot of traditions that straight-up contradict the bible or are based on pagan traditions in Christendom. This is even true in Judaism, the Talmud records the oral traditions of the Pharisees and includes things never mentioned in the old testament (the Tanakh). In Mark 7:1-13 Jesus called and scolded the Pharisees for teaching man-made traditions, like “Netilat Yadayim” (ritual handwashing) as if they were commands from God. So Jesus is against glorifying traditions that are not instructions from God in scripture. Only scripture is inspired by God. So it’s best if Christians avoid accepting things as if they are backed by biblical authority when they aren’t stated explicitly in scripture or shown by physical historical evidence, especially if contradicted by scripture.

Jesus had brothers named James (Jacob), Joseph (Joses), Judas (Jude), and Simon, as well as sisters. His siblings are all mentioned along with their mother Mary in Matt 13:55-56 and Mark 6:3 when the people in Nazareth were scoffing at him. His brothers and his mother Mary were together with the apostles and 120 believers in the upper room in Acts 1:14. In addition, Matthew 1:25 says Joseph refrained from having sex with Mary until after Jesus was born, so the bible makes it clear they had sex afterward.

Some say Joseph was a widower before marrying Mary and his children are from a previous marriage, and while it is possible he did have a previous wife and children from a previous marriage, we aren’t actually given that information in scripture so it is in speculation territory from a scriptural point of view. I’m not against the idea that Joseph had a previous wife and children, but I don’t think it is necessary to believe in order to follow the bible since it is not in the bible. Let’s say Joseph did have a wife and kids before Mary, it still says in Matt 27:56 that James and Joseph were Mary’s sons, so she had these specific sons herself, after Jesus. Jude and Simon are not mentioned so they may not be her children, so they can be stepchildren from a previous wife. However, James and Joseph, were for sure Mary’s children even if the others weren’t. Some people will take the view that Mary adopted them, to which the question must be asked, why exclude the others, what is so different about these two?

Also since the text doesn’t say stepsons, biological children fit in the context. The concept of stepmother is only referenced in the bible 3 times, twice in Leviticus 18:8, Leviticus 20:11, and 1 Cor 5:1, and none of them use a literal word for “stepmother” but rather they all say “father’s wife”. Back then, in this culture, there was no concept of “stepmother” that is equivalent to the way we use it today. Women that fit these roles are always only referenced in their relationships with the father, not his sons from another woman. Example: Jacob had 12 children, from 4 different women. Joseph’s mom, (Jacob’s favorite wife) Rachel, died after having Benjamin (Gen 35:16-20). Does that mean one of the other women like her servant Bilhah, her sister Leah, or her sister’s servant Zilpah, became Benjamin and Joseph’s new stepmother? No, they have a mother who is dead, and even if Rachel’s servant Bilhah raised Benjamin, that doesn’t make her his mom. The definitions of family relationships were more direct in that culture than how we view things in most Western nations today. Leah was simply their father’s wife, and the other women were their father’s concubines to Benjamin and Joseph.

Matt 27:56 and Matt 1:25 together suggest that Mary had two sons. So there is nothing scriptural that says she remained a virgin after Jesus was born. Meanwhile, Matt 1:125 says that she stayed a virgin UNTIL Jesus was born: Matt 1:25 (KJV) And [Joseph] “knew her not” TILL she had brought forth her firstborn son: and he called his name Jesus.

Stephanus Textus Receptus 1550: καὶ οὐκ ἐγίνωσκεν αὐτὴν ἕως οὗ ἔτεκεν τὸν υἱὸν αὐτῆς τὸν πρωτότοκον· καὶ ἐκάλεσεν τὸ ὄνομα αὐτοῦ Ἰησοῦν

The word ἕως (heós) is translated as until or til because in context it functions as conjunction.
Greek Lexicon on Matt 1:25
Heós (ἕως) – Concordance