When I went through this many years ago I found that if I put my son in his high chair and gave him snacks, cheerios, etc. healthy things then I could feed his sister. I would also put a toy or two on his tray and have Sesame street on. It seemed to work for the earlier feedings. The afternoon feeding he was always taking his nap. But her hunger time seemed to be when he was napping and it was just coincidental?
Good Luck, I know it's frustrating as you don't want him to feel second fiddle, but your new little one needs so much extra care in the newborn stages. I also would have Jesse bring me her diaper when she needed changed and let him pull the tapes. Just anything to get him involved with her and helping so he felt protective of her instead of jealous.
They are 29 and 24 today and bonded extremely close and I truly think that is why.
Sincerely
P.