The twin brother story is easy to check out: Ask to see them together! Seriously. This must be devastating. Someone in your situation often feels disoriented in two ways: First, there is the betrayal by the boyfirend. Then there’s wondering how you could be so completely fooled for so long. Both are very, very hard to deal with.
I don’t know why this man attempted to live a double life. I do know that it is pointless for you to try to sort it out so that you can stay with him. It’s unlikely that you will ever trust him enough to relax into the relationship – even if he does leave the wife to be with you. A man who is so casual about leaving his children now is likely to do it again. And it’s very hard to know when a liar is ever telling the truth.
I’m relieved that you’ve only wasted 8 months of your time with this guy. Learn what you can from the experience and be more cautious the next time around. At 25, you have plenty of time to find someone who is worth your love and caring.
I wish you well.