The Virginity of Mary.
Any doubts about Mary’s permanent virginity can be dispelled by her words to the Angel Gabriel ‘how can this come about since I know not man?’ Luke 1:34. If Mary had intended to have children then these words would not have been necessary. But she had made a vow of virginity and which is why she asked Angel Gabriel for an explanation.
Our Lady, Mother of God, was ever a Virgin.
The Biblical mention of ‘brothers and sisters’ refer to those living in the same community but not to siblings. When referring to his associates St Paul frequently mentions ‘brothers and sisters’ with no suggestion of a close relationship.