Picking bridesmaids

So, I’m not engaged yet. But my SO and I have been talking ALOT about it and it’s definitely happening sometime soon according to him.

Anyway, so I’m stuck on how to decide who to make my bridesmaids. I have 4 sisters and my SO has a younger sister.

2 of my 4 sisters I’m closest with, the third not so close but grew up with and the 4th not so close with and barely grew up with (only seen once a fortnight).

Sister number 4 didn’t have any sisters in her bridal party because she didn’t want to have to pick and choose or have another sister feel upset for not getting picked. Which I can understand and that’s fair enough, but from my view she really was closer with her friends then any of her sisters anyway.

The 2 sisters I’m closest with are the ones I would want to be 2 of my bridesmaids. But I’m upset about leaving one out (3rd) and then the other (4th) too. Then, I really want my SOs little sister. She’s only 12 and probably will be 13-14 by the time we are getting married. Then I’ve got like 1 potential friend I would consider being my bridesmaid.

I only want 3-4 bridesmaids to keep costs low and I definitely want my SOs little sister as one of them. How do i pick? Which sister? All or none? My friend?

Is it okay to have 2 sisters and not the other 2? Is it okay to have 3 sisters and not the other sister?

The 3 sisters I grew up with just get me. They know me like fully and we have done life together. One is also my twin sister. How do I choose?