What does the original Greek say?
Mat 1:2 Abraham begot Isaac. And Isaac begot Jacob. And Jacob begot Judas and his brethren. (DRV)
compared to:
Mat 1:2 Abraham was the father of Isaac, Isaac the father of Jacob, and Jacob the father of Judah and his brothers. (NASB)
In the first context it would make no sense that Judah begat his cousins.
The verse I quoted from Mark is interesting in that the word sister (Greek: adelphe) is included in the text and is the feminine version of "brother" or "brethren". If our Lord Jesus was referring to "cousins" (or "brethren") there is no feminine side to that. Our Lord Jesus also contrasts the two by saying:
Mar 3:35 "For whoever does the will of God, he is My brother and sister and mother."
I can only conclude the reference here should be accurately translated "brothers".