AIBU? Or is my mum?!

I think from your DM’s POV, she’s just trying to keep you together as siblings. I know when my DM was alive, I used to see my siblings at her house, but since she’s died, I see my sisters much less often (although we speak almost daily) and my brothers only at special family gatherings. Perhaps your DM worries that you and your sister are so close that DB will get pushed out? But you are not being at all unreasonable, OP - perhaps tell your DM (gently) that you’re not cutting out DB, but just by the nature of things, you’ll always be closer to your sister, and it isn’t for your mother to interfere between you and your brother.

Although you're right I do see where your mum is coming from and thats not to say you've done anything wrong or should change anything I can just understand your mums perspective

Is your brother telling your mum a different story, perhaps?

Next time she mentions it, call him when she's present, and see how he responds. If he responds with indifference, she'll be reassured. If he gets upset because hes not being included, you'll be wiser for it.

My MIL was a bit like this with her adult children. Two siblings drifted further and further away, and she became rather fixated on ensuring the sibling through kept their ties. Unfortunately, the two drifting siblings have ended up really not wanting anything to do with their other siblings for various reasons – it's not great, but it is what it is. This caused MIL great distress, and I can understand why. She's had a very fractious relationship with her own brother which as caused pain to their mother. MIL also went through an awful divorce which caused many years of pain and rifts.

It was 'annoying' when she tried to dictate so much, but I could see it as also a very sad circumstance and how distressing she found it. She doesn't do it anymore as she has accepted what has happened.

It's not really your brother's job, but maybe he could have a word with her and tell her is 100% happy with the level of involvement he currently has with you two sisters.

Tbh I wouldn't be sharing too much with m in your case and that or your sis. M sounds a bit jealous that you and your sister are very close.
I doubt your b is that interested in most womens conversations. I for one am not interested in most male subjects and prefer neutral topics.

I don't know why parents interfere in their adult children's relationships. It doesn't help anything and if you're closer to one sibling then it is what it is. All you can do is reassure your mum that your relationship with your brother is fine and tell her to stop interfering. I doubt she will though!

Sounds like your DB might have told your mum he feels left out.

If I happen to be with mum and im messaging my sister about something

Don't message your sister while you are with your DM. It shouldn't have to be this way, but the easiest solution is to put DM on an information diet as to how often you and DS are in touch.

I think mum is seeing you as siblings but in reality you and sister are friends.
I’d say to brother mum is doing my head in. Can you tell her you don’t feel left out and she doesn’t need to insert herself.
I’d probably distance myself from mum a bit see her less, share less.

It just sounds as if she’s treating you as a sibling group and doesn’t want anyone left out. Try not to get annoyed although it does sound quite annoying.

She's still treating you all as children and you are all allowing it.
Have the exact, unique relationship you want to have with each of your siblings. Don't involve your mum in your relationships. Don't text one sibling in her presence. Don't answer direct questions about your relationships directly. Turn every question/statement from her about your sibling relationship back in to a question for her or deflect with a non sequitur.
Stand up for your self and be an adult, she's worried about you all, but she'll worry less if she sees you're all capable adults, not overgrown kids. (assuming she's not narcissistic or has a personality disorder!)

My dm tries this with my db, who I can’t stand, frankly. I’d stick her on an information diet, she doesn’t need to know when/where you’ve been with your dsis all the time.

How does your mum know who you're messaging or what plans you are making? She sounds nosey.