To think my sister shouldn't support my bullying SIL?

They are both equally horrible.
Why are you even speaking to them?
Cut them off. Completely. Block their numbers and their social media.
Ask your husband to do the same or not mention his sister to you ever again.
You tried to have a good relationship with them and failed. That's ok. There are horrible people out there but not everyone is the same.
Focus your energy and time on good friends/people and cut everyone toxic from your life. You will be so much happier.

I have a SIL like this. And the same lack of reaction from DH. Thankfully he has grown th f up a bit now. But, you know exactly what the issue is, she told you... you stole him from her. It's nothing to do with anything other than a weird codependency. DH has two sisters and mother and they have no lives eg relationships (no extended family, colleagues, neighbours, friends, partners, children), jobs, interests, outside of each other.

You are NOT the problem. She is and the way she was raised. You cannot change her and your DH will need to stand up to her for any significant change to happen. In the interim, absolutely avoid where possible and utterly ignore when not. Do not hand her the power to make you feel shite. You are worth more than that.

your husband should be sticking up for you as that is his role, if never let any family member treat my partner like that, doesn’t matter if he has trauma, would he not protect his children either? I couldn’t respect someone that was ok with this. I wouldn’t go anywhere she would be and she would never be invited to any of my family events. Do you have her at your events/in your home?

Sometimes, manners & being polite get you nowhere. Your SIL was unmoved by the letter & flowers. Now it's time to get dirty. Treat her & speak to her with exactly the same contempt she shows you. Bullies get away with being bullies because they rely on fear & intimidating tactics. Stand up to her & keep giving the awful cow a taste of her own medicine.

Your therapist sounds shite so first off I would definitely start looking around.

You deserve so much better than the people around you, including the loser husband.

Stop pleasing others and think of yourself.
Start detaching.
No contact whatsoever with inlaws and your toxic sister.
Look at what you can do to dump the loser husband and carve out a future with your children.
They deserve a mother who is treated well, not surrounded by toxic adults.

Detaching from the loser husband is always a good start and point blank refusing to have anything further to do with his side is a good start.
Same with your sister.
Look at your work and housing situation?
You had a hard start in life.
You deserve better, but only you can give it to yourself.

What is stopping you confronting SIL and telling her to do one? What are you afraid will happen? Can't get much worse can it? Your DH sounds like an absolute drip.

Sounds like you have more people to drop out, OP!

Your DS sounds horrible but you already knew that, hence the low contact. Time for NC?

You've tried with the SIL - she's not interested. Time to back off her, too. She deserves no more than a hello and goodbye from you at any function.

As for DH, he needs to understand that his inability to speak up to his DS has a big impact on you. It's really quite pathetic that he will just sit there and let her go at you when he knows you've done literally nothing wrong.

Your kids don't need to be the next generation to experience this toxic relationship. Go NC with your sister and SIL, it will probably do your husband some good too. Both of you can show your children what healthy family relationships look like without either of these women in your lives.

Get a new therapist. You don't owe anyone an apology. There is nothing wrong with being an introvert.

Just take a step back from this whole shit show.

Stop encouraging your husband to contact his family, shut your social media privacy settings right down, avoid meetups and leave everyone to it.

Grey rock the SIL and ignore the comments if you don't feel like you can call her out. I'd personally miss some in law gatherings and let your husband take them.

Tell your husband if he doesn't stick up for then you won't be around his sister anymore! The kids can see his family when your husband takes them!

Go NC with your sister. Block her on everything!

But all you are doing is trying to be nice to her, that's not the same as standing up for yourself

And?

Stop communicating then she can't

And you need to stop apologising

Your counsellor sounds useless

Go no contact with your SIL and your ds. If your dh isn't prepared to have your back with regard to your SIL just stop seeing her. If she's at a family event don't go, don't invite her to things you are organising. Just stop completely. If people are happy to stand by and let her treat you like this then who cares what she, or they say about you.

As for your sister, what a bitch, but at least you can cut her off with your head held high, you tried to maintain contact but the first time you entrusted her with your feelings she trampled all over them.

Fuck the pair of them!

Then that shows how emotionally immature she is. So on that note I would say complete no contact. Your mental health is worth more than your relationship with her, and especially as if you and DH were to divorce, you would have nothing to do with her again, she wouldn't be family.
Likewise with your own sis, cut her out too because she sounds just as toxic.
Their behaviour towards you is a projection about how they feel about themselves and their own insecurities.

I'm afraid I'd go no or extremely low contact with both Terri and your SIL.

Explain to your husband (who should understand seeing as he can see how his sister is towards you) that you will no longer spend any time with SIL. If he manages to have a frank conversation with his sister and stand up for you then possibly, going forward, you could attend family meetings. Alternatively tell your SIL to fuck off and leave you alone. Either way attempting to maintain civil relationships with either of these people will only be detrimental to you and your well being.

Block them both and delete your social media, it is doing you no favours!

Oh…and sounds like your DH has also had a traumatic childhood so probably has a whole heap of stuff he’s trying to carry which is why he can’t say anything to SIL. People are quick to yell “You have a DH problem” without knowing anything about the DH. If he’s a well-balanced individual with no M issues who came from a loving family with no problems then he could well just be an arse. If he’s come from a family with with problems (and it sounds like he does) then he maybe needs help breaking free from his family too.

Super quick arm-chair analysis…

You and your DH are both the scapegoats in your families.

DSIS and SIL may be golden children, or they’re just bullies (maybe in DSIS’ case, she got validation from your parents for being horrible to you so she learned it was better to join them as otherwise she may get the same treatment). SIL may have been damaged by going to boarding school…did she resent her brother getting all that attention when she was sent away from the family?

They have discovered that they can join forces to bully you/your DH. They will validate each other so they both feel they’re right in how awful you are. They have found their bullying soulmates and they will love it.

There is nothing you can do about it. You are not the problem, they are broken and don’t function as normal people so will not respond in an expected way. They might have narcissistic personality traits and if they do, the only practical thing you can do is read up on things so you know why you have been feeling the things you have (anxious, uncertain, not trusting what you know has happened, continually wrong-footed etc). Once you’ve done that, you sort out your boundaries. Remember, your boundaries don’t require other people to do anything, they manage how you react. You won’t get your SIL to stop being a dick, but you don’t have to put up with it. You walk away. You leave. You don’t react.

Don’t waste any more energy on trying to work out why they do what they do. Take back control and don’t give them any more emotional time. Drop the rope and carry on without their points on in your life.