IMO, Siblings Without Rivalry and "Mom, Jason's Breathing on Me!" are great books to use once both kids are verbal and able to articulate themselves. So, for my 6 and 9 year old, the strategies in those books work well. For a 1.5 year old and a 5 year old, there's just too much of a power differential to have it work. The bottom line of both of these books is that much of sibling rivalry is a grab for attention -- from the parents. And it's true. When my kids' cups of attention are filled by me, they have much less sibling rivalry. I actually find that Playful Parenting makes this point really well, and those are ideas you can implement at any age -- 30 minutes of one-on-one time with each child as often as you can manage it really does help.
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Have you tried teaching your 5 year old to trade toys rather than just snatching? The deal with trading is that your dd has to 'agree' to the trade. At 1 1/2, it's not so hard to get her to focus her attention on something new. At 3 1/2, she's not going to be pawned off so lightly, but you can cross that bridge when you come to it!
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Do you notice a trend in when your older child acts out? Is he tired? Hungry? Bored? Jealous because she got some attention and he 'didn't'? (It's all about the moment -- never mind that he got attention two minutes ago!) If you can predict it (somewhat), the you can try preventative strategies such as engaging him before he attacks her, making sure he's well fed and rested.
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What do you do when he snatches? In our house, you had to give the toy back. If you were anti-social and did it several times, you got sent to your room to cool your heels a bit. (Actually they still get sent to their rooms to cool their heels, but the issues are purely verbal spats now.) There's not a set time, but simply a space to separate you from your victim and let mom/dad cool down. For us, this worked. (And no, they didn't go happily, but they went, and we stood in the hallway to send them back until they had visibly calmed down.) What would work in your situation?