A 4 yr old little girl - had a new baby brother arrive on the scene.
sibling jealousy etc etc, less Mum and Dad time and 4 months after the new arrival, Dad ups and leaves.
If according to the OP, the relationship was bad, then the poor girl would have picked up on that aswell, but then they were having another child together - um.
She was 4.
Robot - he chose to break the relationship, whether she was abusive or not - we do not know, a non neutral party is making a statement. Who knows whether his body image issues are long standing or not?
Sorry 2 months break between relationships in the context of a marriage and children is not long and in this case it was not two months. "He left his wife to be with me" - sorry you are the OW, end of done finished. Whilst they may not have slept together for 2 months, he hopped out of his marriage into another relationship - pdq.
His EX is pissed off with you - no, can not believe it! He left her with 3 kids, one a 4 month old and from your account, had been spending time with you for those 4 months anyway. I would be pretty annoyed and there would be very few of us here, in those circumstances that did not cry, get upset, say something - maybe we should not infront of the DCs.
The child is being normal, she knows you are not her Mum, why can she not say it, with out you getting upset. Sometimes she wants a cuddle sometimes she does not - again normal behaviour.
Sorry it comes across as you feeling guilty and wanting to assuage your feelings, the child has every right to express her opinion and knowledge of the situation as she is able and you need to give her time.
Your fiance may love his DCs, but lets be honest he did not exactly made them a priority in his life. He walked out on them and straight into your bed.
And those of you who feel that an OW is not guilty in a relationship break down - she knew he was married, she knew he had kids, he chose to break his marriage - but the OW in these situations also knew it was not just about him and her and persisted. Both wrong but different responsibilities.
COI: OW was one of my v close circle of friends, she also knew why we had hit a rough patch and told lies to my then DH about things I had said, done etc. Both guilty