I am very new to quilting. My sister uses a cotton sheet for the middle part of the quilt "sandwich". Does anyone else do this?

I have never heard of that. I have, however, used flannel (washed and dryed) and polar fleece. I know that some people use old blankets.

I use old blankets for middle and sheets for backing ,
but haven't used a sheet for batting.

Are you sure its the middle? For the back, yes.

I'm planning to use a layer of muslin as the filler in my next quilt. I want it to be a summer quilt and I thought a layer of batting would be too heavy.

I've never done this before so I'm glad you asked this question. I'm anxious to hear what you all have to say.

For summer quilts, I have used only the top and backing, no batting.

I have not used a sheet for batting, but have used a flannel blanket for that.

I know of using a sheet for the back.

I have used flannel blankets in the middle instead of batting..they are nice as summer quilts.
i would see what you can find at a second hand shop..

I've never used a sheet for the middle, I've used them for the backing though.

I used flannel once for the middle layer of a table cloth. I think that would be good for a summer quilt or muslin. I've heard that you shouldn't use a sheet for backing. Something about too high of a thread count. Anyone else?