Wow sounds like my house!
I have three girls, one almost 20, one almost 18 and one who is 11. My husband has three children, 14,12, and almost 10. In our home when they come for visits-- (they are not that often because of the travel time, but are often extended by the mom) My troubles are with the youngest.
He has damaged furniture, vehicles, stolen, lied.
The solution to my problem and will be a great help for you is love and logic. I had already taken the class a year ago. I took it 2 more times they say the first time you are just getting used to the idea and the second starting to get it and implement some of it, the third is more of a bring it home I think I can do it.
I was raised more of a drill sargent and my husband is a helicopter, he likes to rescue and drives me crazy there is no consequence.
Yes blending two families is very tough, my middle daughter doesn't want to have anything to deal with him. She went to a boarding academy for 1/2 year and then came home. She is in the public school for the remainder and has chosen to return because of the atmosphere at school, the kids are not bragging of doing drugs or sex or showing off the babies they are having. She loves God and wants to have positive friends and look forward to home leave, she is a sophomore this year.
I told my husband because he had cut up grandmas couch cover with a pocket knife, (she lives at our home) He will not be allowed any knives(he loves them!!) He will go to a church members home and have to do some free labor to work off the replacing of the couch cover.
This is supposed to happen this week, so I am holding my breath because my husband doesn't follow threw and I am generally the bad guy. Not anymore with this one I will not do his meds anymore, Nor be the one telling him to get his homework done, his mom is a teacher and in the past has sent it in the mail as she extends week after week. Then complains after she gets them back that they were here too long and he did not get his homework done.
I don't have a magic answer as I could use one myself,
but check out love and logic online.
I found you can take the class through the foster parent website and it is free. www1.dshs.wa.gov/ca/fosterparents/ongoing.asp
Look for the books in your library.
Good luck mother of 4 girls and 2 boys. My 3 girls at home and his eldest son lives with us too.