Top table challenges...

We have similar issues! We have decided to just have us, our MOH, Chief BM and our 2 best men, so 6 in total. So you could have your sister and niece, the 2 best men and you two. for us, this was far less stress than choosing which parents sat with us and is far easier than splitting the steps and biologicals parents as I don't think that would go down well, but if you think they would accept it then maybe just go for you and the biological parents. The other option is a sweetheart table and is just the 2 of you sat where the traditional to table would be. Finally, you could have no plan, a fried did this, they just told people to sit wherever and then when they entered they just sat on the table that had the 2 spaces left! Is really up to you, at the end of the day it is a meal, a couple of hours were people may have to sit with people they don't know but am sure they can be adult about it.

Do whatever works for you - in general, I think it's best to sit people with those they get on with, rather than those they should be with 'traditionally'. So I'd try to put your young bridesmaid on a table with others her age (or if you have no other kids coming, at least with people she knows and gets on well with)

It might be easiest for you just to have a sweetheart table with the two of you. Put each set of parents on a different table. I wouldn't split up the current pairings just so that the biological parents sit together - that might be a recipe for disaster!

We had a similar headache, as our top table was originally going to be uneven too because FH's dad has a partner but mum doesn't. We've kept the BM and MOH on the top table (no partners), but to even out my side we've added my best friend who's role is to officiate the ceremony - do you have anyone who's important to you that you could add that wouldn't result in the equivalent from your partner's side being added? Your sister that you mentioned in one of the comments perhaps?

Alternatively, do you need to have your people on your side and vice versa? Could you switch it up so you stay in the centre?

Traditionally there's alternating sides anyway, because the traditional order is:
BM, MOG, FOB, B, G, MOB, FOG, MOH

We're erring on:

BM, MOG, O, FOB, B, G, MOB, MOH, FOG, FOG's P

Agree with all the people who say do whatever you want. Why would you have someone at the top table that you don't get on with?!

Unless you desperately want to sit with people I think it's nice to let people sit with their partners, particularly if they don't really know anyone else.

We're not having a top table, we're on the same tables as all the guests and have my bridesmaid, my other best friend and the two best men and their partners with us. My parents are with their family and my partners family are all on another table together. (or three as there's bloody loads of them!).