Why has your DH taken on full discipline for your DDs? Do you discipline them too? There have to be house rules which all dc have to adhere to or it causes resentment. When I have felt similar to you I have detached from the stepkids and concentrated on my own dc for a time. DSD sounds a little like your youngest DSS. She can manipulate DP by being a baby and she will do anything she can to push her brother out of the way and draws all attention on to herself so I understand that resentment you feel.
I don't like feeling this way, and not sure why I feel this way. I have twin daughters(10) whom my husband(their stepdad) has pretty much taken on the full discipline role. I feel that he is strict on them and his younger son(almost 9) does some of the same behaviors that they do, but doesn't get acknowledged or in trouble for. I used to think it was sweet to see him sitting on my husband's lap and spending time with his daddy, but I honestly don't feel that anymore, and I don't know how to get rid of it. The boy is a good kid, but very smart, can be manipulative because he knows he is "the golden child." The twins have behavior issues, as well as being on medication for seizures, so according to my husband "they will get treated like him(the son) when they learn to behave like him. There is another stepson in the family(14), who is withdrawn, keeps to himself. My ex husband was abusive, and gets the twins every other weekend, but they don't like to go. My husband's ex wife gets the boys exactly 50% of the time, and she is one who feels her youngest is all she has left, and wants him to live with her full time. He has been having a hard time adjusting to the blended family, and also wants to live with his mom full time. We have tried to make adjustments(spending weekends with him when the twins are away) but he still isn't happy. We have been living together for 2 years, got married almost 6 months ago. This is taking it's toll on everyone, me and my husband included. I finally was able to admit to my husband 2 days ago how I feel when I see him giving affection or time to his boys, and he says it sounds like resentment, but we aren't sure what is causing it, and we don't know what to do. He used to have a good relationship with the twins at first, and still shows them affection, but only when saying goodnight. Their fighting with each other and disrespectful behavior has made him not want to be around them. I am in the process of trying to find a psychologist/psychiatrist in my area that takes our insurance that doesn't have a waiting list. I have signed my husband up for a marriage enrichment weekend coming up in May. I want a good relationship with my stepsons, and I want my husband to once again have a good relationship with the twins. Anyone have any other ideas or suggestions?