ok it was all too much to read
and though I dont know what its like to have a Stepfather, I know what its like to rebel. I can't say that I agree with whoever said "they may not be smart enough" most children are very smart and maybe he is going about things the wrong way. If their school offers it send them to tutoring DAILY(if possible), have the instructor sign a note detailing what they did and whats left to do. If the school or teacher won't/can't/doesn't if you have the means send your daughter to a place like Sylvan Learning Centers. If her grades were fine in previous years and she didnt jump from regular classes to AP/College classes then there is NO excuse for poor grades. No excuse for laziness, tardiness, being messy, or poor attendance. Talk to their teachers on a weekly basis, do what was suggested earlier and get weekly progress reports. Check in on thier homework and help if you can. Then reward them every 2-4 weeks. Reward meaning they get to go with you and their mother(if shes able) to a movie, or dinner or something family oriented. It sounds like they need to be reminded of what family is. I know they might be rebelling, but its not worth letting them go to find it on their own.
You sound like a GREAT Stepfather, dont ever give up hope on them. They have to understand that they have to use their intelligence and be responsible. I highly doubt your children are "dumb" and cant bring home at least a B. Things may be hard but they will make it through and if that means you keep them home for a while so be it. When theyre older they will thank you.
I also hope you respect them as well. You dont go digging through their rooms, and the like. My parents always came to me with their concerns.. and if they thought something was wrong and I was hiding it, they usually got me without snooping. If you feel the need to seek family councling do so, if you do have insurance and they cover it.. you should do it. Even though you've been around them for a long time they may not recognize you completely as a parent figure. Its good for them and it allows for some moderation with problems they may be having. Sometimes when you can see the problem from outside the box, its easier to find a solution.
Yes its your job to be responsible for your children, but so many parents fail to realize that.