Stressedoutmom, my deepest condolences on the loss of your DS (dear son). I am sorry, too, about your checkered relationship w/ your GD's (granddaughter's) mum. I know you feel there are abuses going on, even though there is no evidence, and it must break your heart. I know it would break mine.
Since there is no evidence or diagnosis you can point to, it's hard for me to gauge what's actually going on between GD and her mother. And I'm not sure where the sexual abuse issue came in (all of a sudden, you mentioned sex abuse by a teenager). But, clearly, there have been tensions between you and the mother, and I imagine that may be why she cuts you out, now and then, though her mental issues may be part of that, too.
If you really feel GD is being emotionally abused, please document everything, and keep trying to get CPS to help out. If the mother knows or suspects you're the one who's reporting her, though, expect her to be angry w/ you. So please thinks carefully and don't take action unless you are convinced. Beyond that, try to stay friendly w/ the mother, so you can be there for GD. And, of course, being as loving as possible to GD, praise her good qualities and encourage her dreams, etc. (you probably already do). Hugs!