Not sitting well?

My baby was actually born January 27th. She doesn't sit well. She does tummy time allll the time. She's very close to crawling. But when she is "sitting", she's very floppy. She slouches down in her high chair, and she falls over sideways when I sit her against things.

My first kiddo never missed a milestone. Not sure what I can do to help. I try to help her sit, but she just rolls over and wants to roll all over the floor.