Perhaps some male bonding is in order, can they find something they are both interested in and spend some time away from the competition of his sister? It really doesn't have to be anything fantastic or expensive, watching a film together now and again would do. The important part is spending time together with no pressures on either of them, something that is exclusively their time.
Try not to take sides, it will only make the situation worse, support your husband at the time (or keep quiet if you really can't) and discuss it calmly with him later if you disagree. If your son knows you will take sides you'll give him more ammunition to throw at his stepdad and his stepdad can't turn around and say you don't let him be a parent.
I think you all need to sit down and talk about responses to situations so they are fair (even if not the same) to each child. If they are going to be different explain to the kids why.If you think this will result in an arguement, go somewhere public to do it and do it over several meetings if nessacary. I guess the fact that his sister was told just two minutes ago is why the reaction to your son doing it is stronger...he already new it wasnt acceptable.
In all of this your son's feelings are priority, put aside whether or not you think they are justified and acknowledge them rather than try to keep them under control.
many thanks Ellie!! A meeting in a public place sounds a great idea as I can't get anyone to sit down and discuss things at home.
My son and partner do spend every Monday night doing different things together already and they enjoy this time..its just when they get home and my daughter is on the scene that the competion and bickering begins which is so annoying!!
I also have a 10 month old daughter who is so demanding that I have little time to spend with the other two. My son sees his own father twice a week and goes on special trips with him to football matches and holidays, so he doesnt miss out on paternal contact.
I understand that my partner will have different feelings for his own children but I am mother to all three and find it hard not to jump to defence.
I feel as if I am stepping on eggshells at the moment as my partner has suggested seperating as the stress is too much!!!
Having read that do you know what springs to mind...that this could be sibling rivalry and your son has worked out he can manipulate the adults and get them feeling stressed and guilty about it.
