I"m working on sashing and cornstones for a 4 patch posey. The pattern has the sashings cut 1 1/2 inches and I can't seem to keep them straight with the cornerstones. I have enough material to make the sashings wider would that make it easier to keep everything lined up? Any secrets out there?

Glue baste them first then sew.

Not sure what you can't keep straight. I like to use sashing and corner stones. I sew the side sashing to the blocks and sew the blocks into rows. Before I sew the sashing row, I double check the finished size of the block to verify the width. I cut the bottom sashing to that size (plus seam allwoance) and sew the corner stones to the sashing.

I take great care in pressing the seams so they nest.
When I attach the sashing row I pin-pin-pin. As long as the seam allowance is consistent, I have never had a problem.

Yes, cut and sew accurately. Press carefully. I pin, too.

As much as I hate to. I would pin it.

Lots of pins to keep it in place

I pre sash all my blocks and never have a problem of seams matching. I cut my sashing along the grain of the fabric, not cross grain, eliminates the streach. pm me if you want more detail.

Pin pin pin....it does take time but worth it

Are your blocks all the same size and square before you are attaching the sashing? If not, you need to do this. Sashing won't line up if the blocks aren't straight to begin with.
Next, are you cutting the sashing to length first? Let's say your blocks are 8 1/2 inches. You need to cut your sashing to 8 1/2 inches. You can't just take a big long strip, sew it to your block and cut it off. You run into the same issues that you would if you did this with the borders.
You can try attaching the sashing and keystones to the blocks, rather than dealing with that big long sashing + keystone strip. To do this, you would attach a sashing strip to the right side of the block. Next take a sashing strip with a cornerstone attached and sew it to the top. Basically you are making a bigger block. Then sew these together.
And finally are you cutting little 1 1/2" squares??? I find this easier. Lets use that same pretend 8 1/2" block size. Cut an 8 1/2 inch strip of sashing fabric, then a 1 1/2" strip of cornerstone fabric. sew the 2 together, then slice this joined strip into 1 1/2" pieces. This is a good place to use spray starch before you do the final slice.
sorry this is so long.....

I don't like handling those lonnnnnnnnng strips of fabric. I sew a sashing strip to one side of the blocks, then a sashing or sashing & cornerstone strip to the top of the blocks.

You can strip piece the sashing/cornerstone units and cut them apart with a rotary cutter.