Can’t pick bridesmaids, DP putting pressure on me to decide

I would go with your best friend as MOH, then your two cousins, his niece and his sister. Don't have any more of your friends other than the one best friend, because then you're into crazy territority for numbers if you have them all, and you're potentially hurting feelings by selecting. The family members are more important. This way you don't get sucked into any ridiculous hen party nonsense either, and can just have a teen-friendly afternoon tea.

It doesn't matter in the slightest if you're not close with his sister. That's not the point. Joining your families is the point.

Is the SIL the mother of the flower girl - in which case it may be helpful to have her as a BM - not to help YOU on the morning but to make the 8yo more relaxed and comfortable.

This all sounds very stressful. I think 1 (the one you've picked) is enough. Boyf/fiancé sounds very intense. I'd nip that in the bud before you get married. Remember, it's just 1 day.

I had 1. A best woman rather than bridesmaid. My best mate. 80 person wedding. About to celebrate our 20th anniversary.

Went to a wedding a few years back with 27 bridesmaids. Very few women at the wedding weren’t wearing the same clothes. (Also 80ish people at the wedding.). It was very weird and they split up 2 years later.

Keep it simple seems to be the best way.

How about picking the most laid back, and those with similar figures so there aren't issues over the dresses as most dresses don't suit every frame.

I suggested laid back to avoid dramas as I'm forever reading about wedding dramas on mumsnet.

I think the two cousins and your three closest friends. You don’t put the groom’s family in the bridal party unless the bridge is also close to them.

I agree with this.

Unless you’re close, there’s no reason to think his sister should be a bridesmaid. At 26, how many women expect to be bridesmaid for their brother?

WRT the 16yo niece, the only issue is balancing her involvement with her younger sister, the flower girl. Many 16 year olds wouldn’t want to be a bridesmaid; they may also feel obligated to say yes if asked. I suppose you could find a different role for her, but honestly, I think it’s fine not too. Her sister is much younger than her.

Id say your two best friends plus your cousins.

This

Are any of his groomsmen from your side?

If not I'd be picking exactly who I want. Your friends.

Id have your MOH and then family so his cousin and sister etc. Simpler than trying to pick friends.

Pick who you want to have. It’s your bridal party. That’s what I’d do if I was getting married.

That's what I would do in those circumstances too

I don't recognise this at all from weddings i've been to. Especially not the having a partner to dance with? Is that aristocratic weddings with ballroom dancing rather than a normal wedding with dancing in groups?

Is this going to be a huge wedding? I don't know anyone who had more than one BM/MOH. I didn't think the gaggle of bridesmaids thing happened outside instagram and American films.

Well as you didn't like the suggestion made by @GennyLec, you're probably not going to like my suggestion - ignore any and all females on your H2B's side of the family. He can have who he wants as a grooms man or an usher but he doesn't get to pick who the bride has as bridesmaids. That's 100% off limits to him (and as a result to his siblings/cousins etc).

You get to pick from your friendship group. Whoever you want but pick wisely because far too many posts on MN are of bridesmaids asking has the bride gone all bridezilla or the bride herself asking if she's going all '-zilla' on the arrangements.

I think it would be mean to have one niece as a flower girl and leave her sister out all together . Unless she doesn't want to do it? Maybe she could at least do a reading or something.
As for the rest of it was me I'd have cousins and his sister. I had my husbands sister as a bridesmaid.

Have your cousins, his sister and niece, and your v close friend.

Definitely not work friends

So six for both.
Him a best man and five ushers.

For you -- two best friends, two close cousins and his niece and little sister. (Other niece as a flower girl also.)
If any of your choices don't accept the roleor are hugely pregnant or would prefer to be a guest with their husband or child etc. choose another from the two other close friends.

You are having such a big bridal party that I think it would be rude to exclude the groom's nieces and little sister.
It is more fun at the reception when you have a large group to eat with.

If you want to have fewer groomsmen and bridesmaids you could swap one or two from that role to being just an usher or they could do the readings or sign the marriage certificate as a witness.. (thinking brother, sister here)

What do you want bridesmaids for / to do?

To look pretty in photos?
To keep you calm?
Help you on the day at all?
To help organise anything such as hen do?

I had 2 adults and a child.
1 adult, best friend, to look after me.
Other adult, cousin, to keep my Mum calm and look after the child who was her niece.

Who do you actually want? I think you should do MOH your closest friend, cousins, second close friend, dh sister. Agree with pp about weddings are to join families so it would be a nice gesture even if you are less close.

Niece does a reading (or if by any chance musically talented plays instrument/recording of instrument while you sign register)

Bear in mind that 5 or 6 bridesmaids is going to cost serious money, dresses, shoes, hair and make up, flowers. You will make a good dent in a grand