I can really sympathise with you, fate hasn't been on your side, each being single and available at the wrong time for each other. You have turned him down at times because you were sensible, first when you were too young and respectful or your sister, when you needed to focus on your baby. Both times these decision were very sensible and showed your maturity. I can therefore understand your frustration that when the time was finally right for you, when you were able to dedicate your time to him and was mature enough to make the relationship work, after he had waited for you all this time, he fell in love with someone else. It's a bitter twist of fate and I think you are dealing with it very well.
Let your gut feeling give you faith. I feel that you guys will be together again, but you won't need to do anything about it, just be patient and wait for the right time on BOTH side comes xx