It is the exact same situation for me!
I married a widower 6 years ago, when his daughter was 11 and son was 13. Their mother died when they were 6 and 8 and I've been in their life for 9-8 years now.
I have a 15 year old daughter, and me and DH have a 4 year old.
DH - classic over compensator, gives the kids whatever they want, is very lenient and his daughter owns his lock, stock and barrel.
Parenting my DSD has been the most challenging, frustrating, difficult thing I did. We never bonded, she openly dislikes me and refuses to have anything to do with me...she is very willful and you can imagine what it was like disciplining her especially as my DH hardly backs me up.
Often I felt invisible and useless. My DD1 felt completely shut out, the two of us like invaders.
It was quite bad at one point, we did a couple of sessions of family counseling but it didn't do much help. Not until I decided to focus all my energy on MY child. I love DSD and still put my efforts in to trying to bond with her, but she clearly doesn't want me around and has made it quite clear, I can't spend anymore time pondering and being frustrated about this because I have two children who need me.
I also really had to show my DH just how serious I am about him backing me up. I'm not a guest in his house, I am his wife, this is my home just as much as his and I am the maternal figure in it.
I feel like widowers want to compensate so much for the deaths of the mothers, they don't even see anything wrong in their kids. How many times did I tell DH that DSD has some issues but he just shuts his eyes I can't count.
But he apparently sees it as perfectly normal that the girls mother died in a car accident and she is going around being the most reckless driver I know.