After the end of DNA Sequence 3, you will be able to upgrade your Stronghold. You upgrade it by purchasing upgrades for the buildings in Monteriggioni, by buying paintings from other cities and by getting clothes, armor and items upgrades for Ezio. You will probably need to do a little of all these things in order to reach 80%. If you go in your Villa and where your sister is, you can get money from the chest that comes from your stronghold or check the book to see how your progress is.