You mean sinlessness of Mary, or which doctrine?
Mary is described as “kecharitomene”, a unique word formation, that refers to her “graced” in the past tense. It is similar to “filled with the Holy Spirit” in the case of St. John. There are also multiple parallels with the Tabernacle that is not to be touched, and the logical reasoning that since Jesus had the rather unusual opportunity to design His own mother, He would probably want her perfect, so perfect she was.
This being said, I understand that there is no scripture that spells it all out, but as Catholics we do not consider that a valid requirement. Further it is possible to read the scripture in the way that suggests she did sin, for example, in losing Jesus in the temple. Ours is one interpretation among several, which happens to be consensus fidelium.
I have observed that
MOST RELIGIOUS consensus issues and processes
have been and are 99.9% POLITICAL in operation beginning to end.
I have no spiritual faith in such operations at all.
Occasionally there are exceptions, mercifully.