brothers/cousins
On the street the ones in the gang are all brothers - If Christ had a blood line though his brothers we would know it - the geneologists would have figured it out and it would be out there
To be perfectly honest, I was raised and educated for many years a Roman Catholic. I don't like getting involved in debates about Mary. Call me old fashioned but I would not debate about my best friend's mom nor even a stranger's mom. So, I usually leave it alone and just present the scriptures and let folks decide on their own. Mary is my Savior and Lord's earthly mother. I am not going to drag His mom into a debate. So you will not get any "Yo mama" debate from me:)
But as you mentioned a blood line of some sort. I just read a paper on dating Revelation and one of the interesting tidbits in the paper was a historical note that Trajan (late first century Roman emperor) called for the "blood relatives" of Jesus Christ from James line. He saw them as potential rivals in a potential uprising. I forget which early Christian historian that was quoted from, but if interested I can find it.