Update. I took out the personal preference part of my letter, added some stuff about trying to teach the boys where they can and cannot touch each other and felt like the letter was sounding pretty good. So I let dh read the letter. He said it sounded good, but still needed some work (my dh's way of supporting the letter). I then said, "well, it does still need work, but I really feel your parents would rather it come from you". HE AGREED!!! So, we may be back to the beginning with this, but dh is willing to write the letter and use what I've written down for a guideline. YEAH!
I also feel I need to protect my kids from staying alone with il's. Any suggestions on that? I should obviously make a copy of the letter and date it. What else do I do? (In case I died or dh and I divorced, etc.).
But you CAN express strong preference, and you can also include in your will a written letter about WHY certain people should not get custody. When the judge reviews the will and determines custody order, he will generally take your feelings into account unless there is a good reason not to.