Hi, Greg,
This is a perennial problem with regard to how to control where Kindle books open. I don't think it's a Sigil problem
per se however.
If you look in the Kindle Formats forum, you will find endless threads and proposed solutions, none of which (AFAIK) are general solutions. If you can find the answer, you'll be a hero until Amazon changes its algorithm (again?)
You'd think, if you are taking the path epub -> mobi via kindlegen, that it would be controlled by the "text" semantic, but apparently not.
IIRC there once was a theory that the Kindle book would open at the first chapter after the HTML TOC that contains text. But that doesn't always hold true either.
My take on it is that it is determined by KDP, and the book builder has little to no influence on it.
Just my $0.02
Albert