Your concerns seem to be mainly 'spiritual'. You are correct that marrying almost anyone could get a pass in the bible. And when God's laws are in conflict with man's law, what does the bible say? And as far as that goes, why not extra wives? Or 'auxiliary' wives (concubines). There isn't any biblical rule about extra marital sex either, just don't make it adulterous.
If I found an appropriate partner, I would not care what their relationship to me was. She could be my sister, any color, hell, she could be from Mars as far as I'm concerned. God doesn't care either. He has only one rule that applies here:
Love her but don't put her before God in your life.