The Catholic Church does not and has never, deified Mary. Read the Catechism or any encyclical ever written about her.
Jesus having brothers and sisters wouldn't take away anything from Him, but if it's not true, it's not true. There is no evidence of it. Mary and Joseph agreed to marry, with him knowing that she had taken a vow of chastity. It was a marriage of convenience, probably so his kids (he was a widower)would have a mother. If Mary was planning on having children, why would she say to Gabriel "How can this be, since I know not a man?"...she was already "betrothed" to Joseph, which in the Jewish faith, meant married!
why would Joseph want to "put her away quietly" instead of just letting people think it was his child? He obviously loved and respected her.