With main meals if my daughter doesnt sit down then I take the food away then try to get her to sit again and just repeat it until she understands she has to sit down to eat but you jjust have to keep trying each meal time so she understands.  Of course it doesnt really help you if the boys arent setting the right example for your daughter. Highchairs are really helpful aswell.

Good Luck

After reading the other response to this question it occured to me that I was talking about snacks not dinner time.  I don't take food off my kids during the main meal.  If they won't sit they get time out.  Then they can come back to they table but they must sit down or they get time out again.  Just tonight I threatened to put my 3.5yo into a high chair if he didn't sit down.  It worked ... this time!

I do it.  It only takes a couple of times before they learn that they have to be sitting to eat.  I don't enforce it too much out side.  My eldest took to this really well and even sits when other children are running around.  My youngest (20mths) is still learning.  We probably aren't as strict on him as we should be.  I think it was being consistant with the first that paid off.