I did a pirate themed birthday party for my 4 year old son last year. We did things on a budget but all out at the same time. For the invitations, I bought parchment paper cardstock, designed the cards (I'm a graphic artist) and printed them at home. Then I burned the edges. I mailed them in semi-see-through envelopes. The invitations had pirate wording of that I had rhymed and in the background was a map drawn.
Decorations, I found some at Party City, some at The Dollar Store and some at Party Co. We found balloons that had skull and cross bones and we had them inflated that morning at a local store. We made bandanas. We bought fabric that was like they would wear back then. We then cut it in the right size and have one bandana and eye patch for each kid upon entering.
For the treasure hunt I bought little things like Wikki Stix at AC Moore, 4 pack of Crayolas, pirate pencils, erasers, etc at Party City. We put everything in ziploc bags since the ground is wet in the mornings here. We did a Pin the Patch on the Pirate game (keep in mind these were 3 and 4 year olds), too.
Our cake was a Costco plain one and we had bought the pirate decorations of another cake and decorated the cake ourselves. You are lucky that your sister will make the cake. I bet it will look awesome.
I hope this helped. We did this last March and some of it I'm forgetting, I'm sure.