Satin? It might not have been the best choice but I'll bet it is beautiful in this quilt. I can understand why you used it for the "look".

It's possible that your side triangles are a little more or less than precisely 45 degrees, and that could account for the shift. You could check that by placing a square ruler in the corners of the triangles to see if the seam lines veer off one way or another. It would not be hard to correct if that's the problem. It's very attractive, and if it's a lap quilt rather than one to be hung on a wall or covering a bed, the slant will not show up. Stitch678's suggestion to block it makes sense to me, and I absolutely agree with those who said not to stress over it. It will quilt out much better than you're expecting right now.

Editing to add that if those edges are on the bias, that's a big part of the problem. Whether they are or not, you could use a technique from garment sewing called "easing" to bring them in to square. Cut both side borders the same length based on the average measurement from across the quilt. Then pin first at the ends, then the midpoints of both the quilt and the border, and then the midpoints between the earlier pins until the fabric is evenly distributed. If there is a lot of gaping between any of the pins you may have to make a running stitch to gather the fabric evenly along the length. The idea is to get a very gradual distribution of the fabric.

It's a beautiful quilt top. I'm sure it will be loved and admired. We all get lessons all the time! Enjoy the process. Quilting keeps my brain working like no other thing can. Love the adventure it gives me. Hope you can too!

I don't know much about the quilting process. The couple of small quilts I've done, I've started at the center and worked my way out to the edges. Anyway, I got to wondering if there's something that could be done in the quilting process that might help to skew it back in the right direction?

Jim