Thank you for your post OP, yes the resentment is real and it's hard to put your finger on - it's just part of the set up really.
For me I have a really good relationship with both my SC. and DP is a positive role model for my DC.
I have been having counselling for over a year and it hasn't helped- I think I'll need to openly talk about my feelings of resentment and see if she can help me somehow.
For me the resentment is that the older SC lives with us full time and my DC are here only 50/50 with their dad.
Also it's my house that DP and SC moved into, so everything that gets touched or used is mine, rather than 'ours'.
Finally DP works from home while I work full time from an office and have a commute to work.
So he is at home the whole time in my house with his child, while me and my dc feel pushed out of our own house.
He also worked in my bedroom, and when his son came home from school each day he'd lounge on my bed talking to him.
Sorry I don't mean to hijack, I guess I just mean that even if you like SC and you're nice to them you can still feel resentment. What I don't know is what to do about it.
Although I would have thought that since the SC are siblings of your DC that would make it easier? Do the children get on ok? Or is the age gap too large to really interact?