I'm not sure there is anything that can be done about this - but just feel a little sad on behalf of my DC - aged 12. He's wanted so much to be part of my DPs big family but despite years together now, he knows that they just don't care about him very much.
We've been living with my DP and my DSDs for 6 years. It was just me and my son for several years, and he is very warm hearted and sociable. He was really excited about moving in with the step family, and started calling all of his step siblings his 'sisters' straight away - he's the youngest. He was happy for me to have a partner too, and took all the adjustments in his stride - not an easy thing for him, being a boy, moving in to someone else's house, being youngest.
It hasn't been easy, a bit of a roller coaster! Some of the siblings were mean for a while which I had to step in to resolve, but there have been a lot of fun times and the youngest two and my son always play together. My DP and him get on well too on the whole. Yet it is hard as there is always some action that just reminds him that he'll never be part of the 'gang' - from my DP, from the 'step siblings', and part of me feels it is a real shame. So much time spent with the other family and he does feel real affection for my DP and the siblings but they just don't back. Sigh! I guess if it hasn't happened now it never will.