Your husband knows the boundary.. your younger son and SS14 are not to see each other.. period.  His parents should know that as well... and should have told SS14 that they could not host him t his weekend due to a prior commitment.  I am not saying that this means they are preferring BS7... but they accepted an obligation with HIM first.. so SS does not get to override that.

DH needs to tell his parents that the boundary is the boundary.. they cannot host both kids.. and that they had BS7 first.. on the agenda.. so if they are hosting older son? then his younger son can't come.  

We aren't talking about average sibling rivalry, this was worse. Way worse and  it looks like you are going to be the ONLY one to put your bio first.  Stick to your guns.  Do not fall for the gaslighting.  Your SS is sick and needs help.  He's not getting it from what you wrote so I'm on your side, they are not to be near each other.  SS is an unproven entity.  4 months?? I'm livid for you.  I'd seriously question DH's sanity.  

And so do his parents.

You will need to hit him over the head with the clarity of the truth of things.

And you are NOT nothing. You and your BS are EVERYTHING.

Get an RO/PO on the pervy Skid keeping him away from your BK.  Take it out of your SO's clearly poor judgement hampered decisioning and out of your IL's hands completely. That pervy Skid should have the Scarlet Letters  PERV indellibly attached to him until further notice IMHO.

Show me yours and I'll show you mine is not an abnormal part of young children growing up together.  However, a teen perving out on a single digit aged child... nope.

Perv-boy needs to be segregated from your child and the rest of the family should not have the ability to question that or facilitate Perv-boy being anywhere near your child.

PERIOD DOT!

This is not a forgive because you are the adult thing. This a SO needs to pull his head out of his ass and protect young children from his Perv-boy failed family spawn.

Grrrrrrr!

IMHO of course.

There are some things that can not be tolerated even once. Much less adults facilitating repeated risk to a victimized child's wellbeing. Even when it is their child that is the one who needs to bear the never ending consequences for their chosen behaviors.

Protect your child.

I don't know that I could ever forgive or forget of any of my DD's brothers did anything remotely close to what your Ss did to your DS. Don't feel forced to have a relationship with him, because I wouldn't either! That kid has issues and he needs to be as far away from your BS as humanly possible. 

My older son  never did anything like that to his brother. They fought and wrestled and broke things, but never behaved like this deviant asshole. I think this situation sounds set up by your MIL and I would be suspicious if she ever asked for your son to stay again. I can't even think of a word bad enough for your husband.

This kid sounds like a pervert far outside of the normal level of pervert behavior expected of a kid. He sounds like he either can't control himself (most kids save that kind of talk for their friends in the locker room, and have the sense not to post it to social media accounts their parents can see), or he has been parented so badly that he really thinks it's normal. The fact that your DH says it's normal makes me think that in their family, maybe it is. Ew. 

You can't forgive a pediphiloe. He's damaged. Don't forget what the kid is and protect your son. I hope you can proetct other children as well, if the opportunity arrises.

Your out-laws (NOT calling them in-laws) are toxic garbage. Their despicable behavior towards you for years is enough to prove to me they have zero place in your DS's life. How they treated your daughter is just icing on the crap cake. 

"...ds is not really bothered by it..."

I'm thinking there is something more to this and maybe DS is not old enough to articulate exactly what's going on or the 'vibe' when he's over there. For your H and his parents to quickly move past something so serious is terribly disturbing. 

Don't let DS go over there. They can eff off. It would be no surprise if they're filling his head with lies and BS about you. DS is your priority. Protect him. {{{hugs}}}

This is just me being nosy but ... what does your daughter think about SS? Has she ever mentioned any dubious behaviour towards her in the past? 

system and keep him out of your child's life.

NOW.  Have charges been filed? Has CPS been called? Has an RO/PO been initiated?

If not why not?  DO IT NOW!!!!

As I stated in your first post. And get ready to PRESS CHARGES. Do NOT allow BS or BD near that sociopathic violent turd.

BLOCK BLOCK BLOCK.

I think, for me, it boils down to one thing: protecting your child from a predator. 

You need to repeat the word "no" over and over to not just your spouse but your in-laws, your SS, and anyone else who wants to put your child in harm's way. 

"No. I will not let my child be near SS"

"No, you won't have my son at your house w/ a known predator"

"No, I will not back down" 

"No I will not let it go" 

"No I will not agree with your parents so you can let them walk all over you. If this child means so little to you that you are willing to appease your parents by endangering him then you are not the man I thought you were, you are not the pare3nt I thought you were and I cannot trust you with my child" 

You absolutely have absolute say of where your son goes, when he goes and who he is with. And SS is not on the list. It sounds like SS is broken,  possibly beyond repair and either way it's not your problem to fix.  Your responsibility is keeping DS safe and that requires keeping him away from SS.  The simple answer is tell grandparents that DS cannot be in the same place as SS for known reasons.

Reporting to CPS might help...they can at least maybe alert SS's school and make it known to school counselors that there's a reason to watch his behavior and take any reports of misconduct seriously (they should anyway). 

I think DH needs to understand you mean business and making a police report is one way of keeping your child safe and maybe opening his eyes to the gravity of the situation. 

As a little girl of 11 or 12, my babysitter took me and her son, the same age as me, to her cousin's house.  Her cousin had a son a few years younger that I was and this kid said sexually explicit things to me and grabbed at my chest....I was mortified and scared and embarrassed.  My babysitter's son smacked at him and told him to knock it off. I didn't want to go back to the babysitter but my parents didn't have another option for me while they were at work. For a while, the babysitter didn't take me back to her cousin's house so I wouldn't have to be around the kid,  but after a while it was back to the same old same old. I just wouldn't be with that kid when an adult wasn't around.... And now as a 40-something grown woman I can tell you I grew up avoiding that kid my whole life and it made an impact on me that won't ever go away.  Dig in your heels and know this is a hill worth dying on.