First of all, it's different when they're your own children. Second, part of it depends on the kids' ages. My husband I and decided to have our children close together in age (2 years or less) for a lot of reasons. The main reason was so that our kids could be close friends as they grew up and so that our daughter wouldn't remember being an only child. Our son was born when she was 18 months old and she's never had a problem sharing anything (food, toys, attention, etc.).Obviously, that doesn't help you a whole lot since your daughter is already three. The only thing you can really do is talk to her. Explain the situation, why you'll be having her cousins over every day. Tell her that while you have to give them attention it doesn't mean you love her less. Giving her more one on one time only reinforces the idea that she doesn't have to share attention, and that's the opposite of what she needs. When you watch your nephews do your best to have activities that include everyone. Play games and do activities that involve taking turns and sharing.Dealing with more than one child is a lot of work whether they're your own or someone else's. It just takes patience and some creativity.