L.D.
I have been teaching middle school for 30 years, have a masters in counseling, and am in a blended family. I have been where you are. A few things that helped me:
1. Get in touch with your own guilt feelings and then throw them away. They are not doing anyone any good and are probably causing harm to you and your sons. If you have made mistakes then learn from them and move on. Stop your thoughts when you start to process guilt feelings. (No, this is not impossible to do, I did it and it works.)
2. Sit down and create some ground rules for the entire family. Begin by making a list of problem behaviors you anticipate and write those down, then make a general statement that is a catch all so when the boys come up with a new problem behavior you have a category for it. Determine consequences for these behaviors. An easy one is to label them all as 'Distrubing the peace'. I got this idea from a great magazine article. Anytime there is conflict do not investigate simply declare both boys guilty of disturbing the peace and punish them BOTH in the exact same way at the exact same time. (I used time out--even with my daughter who was 13. It worked. If arguing continued or if they resisted the punishment then the time out period was increased. Be sure to assign them specific chairs in separtate rooms and take away all entertainment. I used the dining room and living room so I could keep an eye on them.)
3. In your plan you need to also include rewards. Find out from each boy one thing that he would like to do/have as a reward for changing these negative behaviors. VERY IMPORTANT put in a point keeping system and use it. Simply something like have them both start with 10 points and then deduct one for every problem during the day. (Make it so they can also earn back points for designing and engaging in cooperative play activities together.) Put a paper in the kitchen with each the number 10 on it. Every time there is a problem then then one point is subtracted. It is best if you make this a team thing rather than having the boys have a separate set of points. This way they have are jointly invested in working together. Dr. Phil has a simple generic contract online to help with this system.
4. Recogninze that this system is only dealing with the behavioral side of the issue. Both boys probably have feelings that they do not understand. Counseling would help but does not always work and can not be pulled off last minute before the holidays. Likely they have issues with competing for your love and attention. Avoid your personal guilt trips and show them both your love and approval IN ABUNDANCE. However, do not let this love and approval in abundance expression interfere with the point system. Your message along these lines should be simple. I love both of you and your behavior right now is not OK--that simple phrase,"NOT OK"; so here is the consequence. If you get caught up in the guilt stuff you are only going to allow them to manipulate you. When children manipulate the adults in their lives it makes them insecure. This insecurity comes from their feeling like they are in control of you instead of your being in control. Kids like feeling that someone else is in control even though they will tell you differently.
5. Best of luck. I am a Christain. I always combine all of my solutions with a healthy dose of prayer.
Hope some of these ideas work for you. Happy Holidays L. in NC