I have a little bit different suggestion since you have plenty of fabric. I really do love the look without the sashing, but also think the blocks would shine with some sashing, so how about this:

Make a center panel without sashing, add a border of a width to bring it up to the size needed for the blocks with sashing and add a 3 block wide border of blocks WITH sashing around the center panel.
You could even use cornerstones in the first border and the sashings to add an outgoing effect of squares in each corner.

***I like sashing, but
***Either way It will be a beauty.

Wonderful quilt so far, the colors are so calming. Your gift will be treasured for sure. Please show us when you have decided and finished it.

Ditto. and you have a darker blue in one of the squares that might work well for the sashing. You could then bind with the yellow or vice versa. No matter what you do, it will look beautiful and she will love it. Photos of it when finished please.

I, too, like it without sashing, to maintain the latticework effect, but a border - even a double border - would bring it all together nicely. It's a very pretty quilt. Agree with Dhanke, too... a triple border would be even nicer. Your SiL will be so pleased with this quilt!

I like it without the sashing. Show us what you decide.

I agree with dhanke,, on the yellow shashing and deep blue borders

I love it the way it is. But, if you wanted something a little different, use sashing with cornerstones, it would make it look a little like an Irish chain. Either way, it will be beautiful.