I'm blind! I'm blind! I'm blind!

Did you
HAVE to actually type those words so I'd have to
READ them? I am dead certain to have nightmares tonight.
Since SIL told you to MYOB and you all want to hold down your Thanksgiving meal instead of tossing your cookies, I'd tell her it
was my own business because it
is my own house. And in my own house, kids that are old enough to order from a menu, read a book, do math homework, and prepare simple meals for themselves are NOT allowed to ask to suckle on mommy's hoo-hahs right in front of God and everybody because it's just not done....at least in my house. Period.
For the record, I don't care about what is done in other societies. This is America we're talking about (the OP was an American, wasn't she?

) and in our society, breastfeeding a 6 and 8 y.o. is not acceptable. Forcing others to watch it is even less acceptable. If you want to question why others find it uncomfortable, it's because they have a clue as to social conventions and have picked up on "it's not NORMAL in our society." SIL is the odd one out, not them.
She's not brave....she's weird.
I'll come out and say it. I'd draw a line in the sand on this one. Someone needs to stand up to that nutcase. And as I often tell DH, "Obviously no one else is going to do it, so I guess it will have to be me." Fine. If she calls your bluff, tell her in front of everyone that you won't allow it. If she insists, tell those kids you think they are too old to nurse and it's not going to happen in your house and let the fallout begin. Since I'd be tempted to call CPS anyway, what does it matter?
Why should teenagers (or adults) have to watch something that freaking weird? Why should they be held hostage by a bully breastfeeder? I'd be tempted to have the 18 y.o. have a bunch of his buddies come over for a viewing with their camcorders/cell phones and post it on
youtube.
Seriously, this would NOT happen in my house. I'll bet everyone is praying someone else will stop her......It's just that no one else has the nerve to do it. So do the world a favor and stop her in her tracks. If for no other reason, for the sake of her children.