This could go bad in many ways, but it can also be great. They may absolutely despise each other and fight all the time. If so, you need to let them battle it out a little bit. They are old enough now that they will probably get sick of fighting with each other and just ignore each other. They are also mature enough to know they are stuck in the situation whether they like it or not. One thing that may help is if they can have their own rooms, since privacy is so important at this age. If they do detest each other, they may find common ground in hating you and your partner for bringing them together. If you can deal with that, they may be able to come together a bit, which will be helpful for them.

You may have a situation where you have a stepson and your partner has a stepdaughter and it is love at first sight. Unfortunately, you cannot allow this, especially if they live together. Telling them this will never happen may push them closer, so be careful how you handle it. Remind them constantly that they are now siblings and it is an inappropriate relationship to have. While you can celebrate the fact that they get along, discourage any sort of sexual relationship between your biological child and your stepchild. If it seems to be a major concern, a family therapist can be of great assistance.