Jesus had no other siblings. The term used for "brother" can mean cousin etc.
Our Lady, from the very instant of her existance in the womb of her mother, was concieved without the stain of sin (known as "original sin" or "the sin of Adam") because although every other person that is born into this world has the sin of Adam handed down to them by their parents, for God's purpose and by His will, God spared Mary of this sin due to the impending position that She was to partake of later in life, hence, any child that She would have bore would not have had the necessary ingredient to be born in original sin because She was not born in original sin, therefore the sin itself nor its effects could be handed down to Her child.
"The term used for "brother" can mean cousin etc."
Correct. The Greek word used for "brothers" in the NT is "adelphoi" (cf. Philadelphia - the city of brotherly love).
There is a clear example of it being used to include a wider range of relatives in the Greek translation of the old Testament in the book of Genesis. Abraham and Lot are referred to as bothers - "adelphoi" - however, their actual relationship is one of uncle and nephew.
Where is that usage explicitly found in the Bible?
Suggenes vs. adelphos. Since there is a perfectly good Greek word for "cousin", why wasn't it used in these disputed passages?