Birthday party ettiquette would deem one goodie bag per invited guest. If the older boys are invited guests, they get a goodie bag. If the invitation included baby sister, she gets a goodie bag. If invitation did not include baby sister, but mom called you and asked if she could come and you said yes, she is as good as invited. If you just happen to know about baby sister and are assuming she'll be there, you don't have to provide a goodie bag for her, although it would be a nice thing to do.
Goodie bags can be as simple or as exhorbant as you make them to be. Some stickers, a granola bar, and a bouncy ball can suffice. Try buying things at the dollar store, if you are on a budget. You don't have to cater each bag to the child unless you want to. Goodie bags are a nice way of saying thank you for coming to my party, and it is acceptable for all the bags to be the same. It is up to the baby's mother to supervise her treat, if you choose to give her one.
Hope this helps!