Would it be in bad taste to ask my friend if her daughter would like to be a flower girl if I’m not asking the friend to be a bridesmaid?

There is no need for her or the child's father to be in the WP, because the child is. And no need for both halves of a couple to be in WP. Make choices of the one person you want, including for flower girl, not mommy daughter package deal.

Or FG and RB parents are not in the wedding. My BM are my sisters and nieces his grooms women is his sister and best friend.

My very dear friend of 18 years was in her brother in laws wedding and her 3 year old daughter was flower girl. All she did was complain about what a pain it was and how overwhelming it was to have Liv in the wedding. She also had a 1 year old at the time. I think if you explain that you understand she has a lot going on she'll be ok. Maybe get her a corsage or something and make her a part of the honorary bridal party. She'll probably want to keep her eye on her daughter, anyway.

FW asked her niece to be in the wedding but not the niece's parents. It's fine. Make sure to ask the parent(s) first if it's ok for the child to be in the wedding.

I was planning on asking my fh's neice and nephews to be in the wedding, along with their father( my fh's brother) but I wasn't planning on asking his wife because one we aren't super close and two I want her to enjoy her time at the wedding, I'm sure it's already stressful enough having her husband and kids in the wedding too!

It's totally okay for the girl's parents to not be part of the wedding party. They can attend as guests