I've been reading this site for a few weeks now and find myself agreeing with alot of what you say. Here is my background:
I have a DS9. His father is in prison and has been for over a year. he has a few more years of time to serve. I was divorced from XH when DS was 3 and XH has always been a POS; rarely came to see DS for visits, never paid CO CS, was a total bad influence and role model to DS. Of course now that XH is in prison he has once again been saved and is promising DS the world when he gets out.
I've been remarried for 4 years now to a great man, ball-less but great. He has 2 sometimes 3 kids from before we married (SS9-that he does see but pays for) & SS5. I say sometimes because DH has a SS5 that has an older brother SS13. DH is not SS13 father but has known the kid for year and years and the kid has a POS father that lives out of state. While dating and the first few month we were married DH did not bring SS13 for visists....so it's not a matter of i should have known before we marrid. We see SS5 EOW and his brother SS13 now comes with him. I think SS13 comes with him to give the mother a break because this SS13 is bad. he's been on felony probation for a year now and has violated it every way he could. He's stolen from houses (grand theft), got failing grades, got into fights at school, tried to burn down the school and recently was caught with drugs for the 2nd time. The probation officer in his case does nothing. SS13 was diagnosed with ADD and ODD and takes meds for it.
Of course we experience what most of you do the double standard of behavior. For example: My DS gets a D on his progress report (first ever) and DH wanted to ground him for weeks. SS13 is a felon and continues to mess up (got 8 referrals so far this school year) and he runs around footloose and fancy free. I am really irriated by this.
In the last few months SS13 has been down right mean to DS9. Aside from the broken toys and stolen stuff SS13 has tripped, hit and called DS9 names. A few months ago SS13 had DS9 up late at their aunts house and started teasing and belittling DS9. the final straw was when SS13 made my DS9 lick the bottom of his shoe. Of course SS13 threatened to hurt DS9 if he told. at first DS9 was really upset by this and didn't want to be near SS13 for weeks. DS9 has since forgiven SS13 although i'm sure there was no apology.
Since this happened i have tried to keep the boys separated. Even finding some place else for DS9 to go sometimes when we have SKs. DS9 didn't seem to mind, he has lots of friends. It's awful for SS5 because he and DS9 play well but i have to be concerned for my DS...no one else is. Also DS9 cannot have friends spend the night when SK are over because SS13 is so mean.

Everytime i bring this double standard behavior up to DH he denies it and of course doesn't want to yell at his kids or they may ~~gasp~~~ be unhappy. I've tried to explain this situation to DH abotu SS13 but he refuses to see it. THis past weekend SS5 come downstairs in tears to tell us he HATES his brother, SS13 was wrestling around WAY to rough and hurt him. Of course still no reaction from DH.
DH doesn't pay CO CS for his SS5, but the BM does keep DH jumping with chores at her house and buying snacks for the kids. i think this is part of the reason DH guilt parents like he does. if the SK are unhappy he may not get to see them and he may have to start paying CS for SS5.
that is my life. During the week it's just DS9, DH and me and we get along well. But EOW our house is a war zone.
My Blending is so fun!!!!