For an FYI about your relationships in the future: Move a little more slowly the first couple of years, even when in love.
The amount of kids' stuff you have moved into his apartment already make me suspect that the guy got scared that you were integrating your life into his so quickly. He was becoming like a second father to the boys without having made that decision, actively, to be their stepfather and what that entails. He was becoming one, just by boys expecting to see him and having their stuff in his place, having not had time to fully consider what it means to be a stepdad.
The fact you called his sister involving her in your relationship is a red flag, to me, that you were pushing the relationship. The first year or so is still a testing time for a relationship. Unless the sister was your best friend before you met him, then this was a no-no. Friendly to sister - yes, in a surface way is OK, moving slowly. Talking to her about your life and relationship drama - no, too pushy. Most spouses and boyfriends prefer that ANYONE else be told about relationship problems over their immediate family. My husband would kill me if I confided in his sisters. It's considered bad form because it requires family drama and civil wars within. Use friends outside his family for personal-relationship support, not his family.
It's also likely his family had been concerned the relationship was moving too quickly, and had advised him to seek "space." They saw a guy fresh from a divorce who had a new date and kids in his life already, and realized that if he is on rebound, he needed to separate his compatibility with you, from being someone who just needed to be in any relationship with anyone rather than being alone.
The space may be good for both of you.