The best advice I received was from an older hospital nurse (50-something and she had 4 kids)...
One leg for baby, one leg for the older child.
Whenever they want your attention, you offer to sit with you and baby and child. It helps equalize the attention received. If baby is nursing, the older child can be there and feel as if they're getting equal amounts of attention. It wore off quickly in our house. It went from almost every feed, to a few times a day, and in a week, DD was pretty over it. I talked to her, gave her hugs, would read a book, etc. This might also be helpful when you're tandem nursing; you can reading a book and teaching manners while the baby is present.
We also encouraged her to help with the baby. Like "holding the baby", which was her putting hands on baby's bottom or back. It lasted like 10-15 seconds, and then she was over it. She'd randomly ask to hold the baby, so we happily obliged to let her "hold him".
They're 25 months apart, but I think these could help. Good luck!