Sirris appears in Firelink Shrine after meeting Anri.
If you give the Dreamchaser's Ashes to the Handmaid, Sirris will start a new dialogue with you and teach you the Darkmoon Loyalty gesture.
After killing Aldrich, a unique Budding Green Blossom will appear in the Shrine Handmaid's inventory with a special notation that reads: "Good-bye, dear Grandmother. I will visit Grandfather, in the Pit of Hollows." Not only does this imply the Handmaid is Sirris's grandmother, but also that Holy Knight Hodrick is her Grandfather. If you follow here there, she will enlist your help in killing Hodrick because he is mad.
If you support Rosaria, Sirris will turn her back to you. Also, killing Company Captain Yorshka will cause Sirris to end all communication with you.