Hi there!
It is sometimes difficult for children, when a new little baby comes to the house, as you well know. Try and get her involved in taking care of her baby brother or sister. After all she is the big girl now, and now she has her own real baby to play with and take care of. Let her help you get the diaper or help you dress the baby and even help you for bath time. Ask her to sing a song for her baby and just let her get involved so that she can feel helpful and included. And let her know that once she helps mommy put baby down to sleep then mommy will have some time to do things with her. Like have some tea time and cookies together, or whatever it is that you guys enjoy doing. And maybe, when the baby is down for a nap, you can have daddy, or granny or a friend come by to stay with him or her and you can go out to the park or do something with her. There are also some nice stories out there about big sisters taking care of their baby brothers or sisters, and it let's them know that even though the little one's are going to stay there from now on, it lets them know that the older one is still special, and that mommy and daddy still loves her very much. Yeah, just let talk to her and love her as you always have, and let her you know that you are always there for her. Let her draw pictures, or read books, or let her do puzzles or build block or she can even cut cookies while you are breastfeeding. There are plenty of things that she can do to keep herself occupied during those times. If she justs wants to be close to you when you breastfeed, then let her. She can sing a song, a lullaby and help you put the baby to sleep together.
I hope that some of this will be helpful.