Is It Proper For My Ex Boyfriend To Marry My Sister?

There is this guy that I have been in a relationship with for three years, though there was no sex between us though. We broke up because of some challenges in my family cause they all felt it was too early to go into courtship then as I was still seeking for admission then, but we loved each other so much.
He introduced me to every member of his family and friends including my family as well, but there was nothing I could do about it. 
My problem now is that he fell in love with my blood sister of which both of them are aware of our past relationship. Am really troubled and hurt caused I feel it's not proper for them to get married. Now the wedding date has been fixed to Easter next year, the family and friends that knew us then called me everyday that I left my guy for my blood sister, I tried to make my sister understand that she can't marry the guy but it got to a point that I just have to let her be because she started thinking that am jealous of her cause she is getting married before me. 
Please ma, I need your advice cause am always hurt whenever I flash back. Help me please to get over it.

Though you didn't indicate your age and the challenges that your family had with your relationship then, but I don't think that your sister is entirely guilty of your hurt and pains. 
I think that your ex though he loved you fell in love with your sister and your sister decided to take advantage of the opportunity and today they're hoping to get married next year. 
Since he feels that your sister is who he wishes to marry and your sister also feel that she doesn't owe you any explanation for her betrayal, please wish them all the best and let them be.
Sometimes there is always something ahead of such betrayals that may not be revealed to you now but will definitely make sense in the future. 
Don't regret over it, he's not the only man in the world and he's not the best man ever. If he could stab you in the name of love, forgive him and move on with your life. 
God is a master planner and nothing is ever a coincidence in his vision for your life so don't reduce yourself to your sister's pedigree or let the opinions of others make you feel discouraged and worried, but return all glory to God, accept everything in good faith, give them your blessings and steer clear from their marriage. 
God will give you your own husband and man that will make you feel fulfilled and grateful in life.