Hi again, I know this is totally old now, but I had another thought. You said you didn't think that this was a sibling jealousy issue, and I agree that jealousy might be the wrong word. However, I think the behavior is really common for siblings that are your kids' ages. In the book The New First Three Years of Life, the author repeatedly makes the point how much harder it is for the older child when the younger child starts crawling than when the baby isn't yet mobile.
That was definitely true in my case. My 3 year old son absolutely adored his baby sister until she got good at moving around. For me, problems didn't start until she started walking, but once the older one couldn't keep things away from her just by putting them up high, his frustration level went through the roof. Oddly, he didn't really take it out on her very often, but he started acting up at school and was just generally more miserable until we got things under control. What helped for me was to at least act like I was taking his side most of the time since the 1 year old obviously had no idea what was going on. I frequently told her, "Uh-uh, it's brother's turn to play with that." Eventually she actually started listening and backing off when we asked nicely, though that obviously took months. Now they're almost-4 and 18 months and they actually play really well together. So maybe there's hope for your kids in 6 months.
Also, regarding concrete examples, I think you might really like the book Children: The Challenge. It's from the 1960's, I think, though I really agree with about 90% of what he writes (just ignore the over-arching 1960's mentality). Unlike most parenting books, it's just back-to-back-to-back scenarios and how to deal with them in ways that are surprising practical and helpful.