I have two stepkids. An 11 yr old and a soon to be 18 yr old. Im also in the process of leaving their mom. They dont blame me as they know how she is..but they also love her.
I learned fast and hard the worst thing to do as a step parent is to try and BE a REAL parent (unless you get em young like babies to maybe 5. Otherwise, do what a real bio parent SHOULDNT do..be a friend. o be sure, back up the bio parent with their rules, etc..
ANd even though they respect me..cuz ultimately, they know I dont take any crap, I have to quote Alec Baldwin...they can certainly be at times, thoughtless little pigs.