Children of 'Retreads': How do I help him adapt and accept?

[LoveBeingAMum7]

My partner and I have been together for about 7 years and living together for the last year. Between us we have 4 children - a daughter at university and 3 boys at home ages 12, 14 and 15.

The youngest 2 are mine and have adapted well to moving school, city and home to be a family. The older 2 less so (although they have only moved home) and the 15 year old is actively starting to rebel. He resents being told what to do, claims not to want to be part of this family - he didn't ask for it.

I think he is missing his sister and also being a teenager, but he is also fiercely keen to have his Dad all to himself. If he has enough, he goes off to his Mum's for a few days but mostly he lives with us. He knows that at the end of the day if he refused to live with us his Dad would go with him - as any parent would.

What should I do? I feel like I am treading on eggshells around him - I consciously do not ask him to behave the way I do my 2 boys as I know he would kick off and it would upset his Dad. Sooner or later they will notice and ask why he gets to behave differently. I need to help him or he could drive us apart!

Advice please!