OP well done on making the call. What did the NSPCC advise?
Remember, you can ignore posts on this thread if you think they are way off the mark, or hysterical knee-jerk reactions. If you take those posts away, how many are you left with which offer sensible, rational advice or valuable 'point of view' personal experiences? Quite a few I should think.
The suggestion that your DH is/will be abusing is NOT unanimous on this thread. Many, many people have expressed concern that it may be a possibility and warrants further investigation, preventative measures.
My understanding is this. Your daughter lived with your parents due to your circumstances. During this time, you met your DH and very quickly fell pregnant. You married and had the baby, dd2. When dd1 was 3 she came back to live with you and your new DH and baby. This was the first time he met dd1 so he went to great efforts to bond with her. He helped raise her and possibily did a lot of dad stuff with her whilst you were caring for dd2. As is common with a lot of men, he enjoyed the time spent with the young child (dd1) rather than the baby (dd2). Consequently, they built up a close relationship and she came to love, trust and rely on him. DD1 was a fairly demanding child, wanting her own way in most things (common with all 3 year olds!) and because you felt guilty about her early years, you possibly gave in to her a bit to much. It became a habit to allow her her own way where possible, such as sitting in the front of the car, lying down with her at bedtime, etc. Now you are finding that you seem to have been 'pushed out' of the relationship with DH and whilst you are not jealous of your dd1 you feel that she monopolises him too much and it is also affecting your other dcs. This is my understanding of what you have told us.
What the posters are questioning is why he allows this to happen and doesn't see it as a problem like you do. Does he has other motives? That is all. It is just questioning his behaviour, as an adult. As you, yourself have done.
I think everyone has recommended you seek RL professional advice and that is what you have done. I'm not sure if you would have without all the advice given here though, so something good has come from your post. Hopefully things will change for the better now.
I for one, would love to hear from you again about how it's going but if not, I wish you strength and courage to face and deal with whatever you need to.