I never really explained it to Seren, just said once that Cally was having milk and that was that - she accepted it straight away. We did have some teething problems when Seren used to throw a tantrum as I sat down to feed Cally but TBH she would have done that if I had bottle fed, breastfeeding just meant I had one arm free and I used to play games with Seren, read a book etc. Our fave thing (and most effective) was to do singsong, Seren would choose and off we'd go lol.
You could have a box with new toys, books etc that he is allowed when you are feeding baby, or a dvd (Seren was allowed to watch Alice in Wonderland if Cally was being fussy). It is hard as I found Seren would always ask for a drink as soon as I sat down so I used to keep some little snacks by me and make sure there was a cup of juice etc so she would have something to eat and drink if needed. Seren also liked to have her doll to feed.
She did ask a couple of times to have a booby. The first time I said no and regretted it as it was pushing her away so the next time she did I said yes and all she did was mouth my nipple for a second then laugh and run off. Then she asked a few times after and I would say "yes you can after Cally has had her milk" by which time she had forgotten. In fact lately when I say "Seren used to have booby when she was a baby" she laughs and goes "no, Seren is a big girl and doesn't have booby".
kellymom has some good tips
http://www.kellymom.com/babyconcerns/ne ... ddler.html
Good luck with it, it is hard at the end but it is so rewarding. Seren strokes and kisses Cally's head when she feeds and loves snuggling in with us. She is also very protective over her sister breastfeeding, and I feel like she will grow up to know breastfeeding is just a way of feeding her chilkdren.