to think this is normal, or should I/we interfere more

[ReadyForGoodNews]

My two teenagers, DD15 and DS16 are always, ALWAYS arguing. The whole fucking time. We can't be in the same room all of us together without them either muttering insults to each other or just plain out right shouting.
We're having dinner, they argue about who's eating what, how much the other has had etc. If we're playing a board game, one of them is always making fun of the other, taking it too far and then it all becomes a huge argument.
DS has form for always criticising DD. Things like "oh you're a failure", "you're so dumb", "look at her, she's pure evil", "you'll never get anywhere in life", "You always fail at maths" and then whenever she says something about anything, he will SIGH sarcastically, mutter "ugh, idiot"... etc etc
DD on the other hand, loves interjecting all conversations with something mean about her brother: "no one cares what you think", "who said we want to know your opinion", "you have no friends", "yeah right that's so brilliant, you think you're so great", "yeah we really don't know it was you who finished the cake"
Really NOT NICE!!! And I really hate it!!!
DH can't handle it and when it gets to a certain point, he gets upset and tells them off, to which DS replies with a very infuriating satisfied smile and DD acts like a horrified victim and leaves the room.
I try to ignore until I feel it's going to go too far, and I either change the subject, or say something like "enough, if you won't stop it please leave the table/room until you calm down".
This doesn't change the fact that the same happens every day every time they're in the same room. They used to have an ok relationship, normal sibling stuff.
We try very hard to never take sides. DH thinks we have to come up with more strategies and be more involved in trying to change this. But it just seems impossible to talk about this with them at the moment. Whenever we try, they think we're taking the other one's "side". UGH!!
AIBU in thinking this is normal teenage stuff and they'll grow out of it?

linoleum That's interesting..my brother and I have a younger sibling with LD/asd and he really struggled with it-was highly critical of her, angry, dismissive etc but it definitely came from a place of upset not being sure how to manage it. Am glad you addressed it it never really got addressed in our house and although as a 35 year old man he is kind to her now he really struggles to know how to relate to her.

Your reply to me was that they are not growing up in an environment where being told you're a failure and other things is overlooked, yet in your OP you said that's what your son was calling your daughter and you tend to ignore it .

She's getting the message that you don't intervene when he is abusive and thereby it's okay for him to treat her like that.

It's awful and will stay with her for life if you don't change your attitude honestly.

Switch off wifi. 10 minutes for every mean comment. Let the consequences do the talking.

Cappucino I didn' t say I tend to ignore it, I said I try to ignore it until I feel that it will go too far, so it was more like waiting a few minutes to give them a chance to realise they need to stop. Not ignoring it in any way. We do tell them to stop if they can't say anything nice etc, and in my OP I did say it starts again the next day, I should've said they do stop when we tell them to, but they start again the next day.

To those who said my DS sounds abusive towards DD, yes we have noticed this too and when we had a chat with them separately the other day, we mentioned to DS how abusive his behaviour was - he hadn't seen it as abuse, and was mortified. We didn't say the same to DD, we told her it was unacceptable to talk to one another like that. Both were set consequences and for the last two days, every time DS is about to say something nasty, he is stopping himself, and saying "sorry sorry I know" and leaving the room or just shutting up. DD too, she opens her mouth to say something nasty then she goes "hm anyway/whatever" and goes on doing whatever she was doing.
I know it's not perfect but still, some results.

To the poster who said to switch off internet for both and they both have to work towards getting it back - great idea. It might even foster some solidarity between them.

I know it all sounds horrible from my OP but it doesn't mean we're ignoring it or finding it ok. As I said, this is new behaviour from them so we're in the middle of dealing with it. Once the behaviour is under control, we will try to understand what triggered it so it doesn't happen again.

That's sad tangled. I'm sure it's not uncommon and it is hard for children living with siblings with SEN because the rules are, or have to be different and it's never as fair as it would be without the SEN. We're working through it, underneath the nasty exterior is my sweet and kind boy and we're getting him back.
Glad you're getting somewhere too OP. Just curious if your DS was ever bullied? Mine was bullied in primary by a boy that would criticise and attack his family, his brother who was just a baby then so we didn't know about his SEN, was the biggest target because DS became popular for his cute baby brother and the bullies parents were going through a nasty separation at the time. The boy knocked his self esteem, so unlike his sister who has bags of confidence, he struggles with feeling embarrassed or critical of his family. As soon as teen hormones hit, the the victim became the bully . Talking through it and pointing out the behaviour is really working for us too. Hope it continues for you.