magwaaf (in SST)
You need to talk to all four witnesses. Your journal will then tell you to gather evidence. The first bit of evidence is the painting and easel found in the WEST tower (you can get there by going to the Arch tower, or something like that, the entrance is next to the mage girl's bedroom), and in the first room behind some crates is a hidden trapdoor. Just keep going, you can't miss the painting.
Second bit of evidence is a box on one of her tables. Open it and you'll find painting supplies.
Third bit of evidence is paint stains on the dining room carpet.
All of this together apparently leads to her being guilty, you can then accuse her.