Long! Sorry!

Background:

My partner has sd7 and sd5 and has been guilty of guilt parenting. We have dd3 and he has favoured his girls, particularly sd5 over ours time after time. We've discussed this, he accepts it and is trying to change it but it's difficult. He left the marriage when ex was pregnant with sd5 and I think that's a big part of why he has a hard time disciplining her and lets her run all over him.