Here's an idea, take photos of your kid, and not an advertisement for a prop
Don't worry about sitting, focus on what makes your daughter special. Does she have a favorite toy or blanket? Does she sleep a certain way? What milestone is she working on? Rolling over? Pushing up?
I say this from personal experience, btw. I got sucked into the "prop with a side of baby" gimmick when my girls were that age. Of the hundreds I took, I only have a handful that are truly full of emotion and capturing what they were at that time, which for me, is why I take photos Just my 2 cents
As mentioned, if you can't get them to sit up, have her lay on her tummy and hold her head up. Just remember to get down on their level.
Here is a shot of my daughters.
You might also consider a Bumbo chair. We used them for all sorts of things, from feeding to photos. They are really only good from the time they can hold their head up on their own, to the time they are strong enough to get themselves out. They can be expensive, but are robust, so you might be able to find a good used one.
Another prop that I sometimes use, is my home made baby stand. It's basically two pieces of wood, hinged together like a sidewalk sign (only heavy duty). I then have an adjustable shelf on one side. The kids stand/kneel on the shelf and lean on/over the top. It's a way to get a head shot really. A parent can hold the child with one hand, from below/to the side, to keep them from falling off.