To find Chrysis at the forest altar, head south east of the guesthouse. As you approach the target use Ikaros to identify the precise location of the altar. When you reach it interact with it to trigger a brief cut scene. You should then head in the direction of the crying baby. As you reach the temple two cultists will ambush you from the bushes. Take care of them and then enter the temple. A cut scene will then initiate and you will be able to question Chrysis about your mother and sister. At the end she will attempt to burn the temple down, with the baby inside. You must now choose whether you want to go after Chrysis and kill her now, or save the baby and have the opportunity to kill Chrysis later.
Note: If you choose to save the baby you will be able to kill Chrysis in a later side quest. Killing Chrysis now will provide you with slightly more XP for the current quest, but less overall XP when taking into account the quest to kill her later on in the story.