Before I even got into IM I wondered if my thoughts possibly contributed to my grandfather's death. I don't blame myself that much though since I'm pretty sure it's true. I didn't know. I'm almost certain he wanted to go as well.
While your loss is much more tragic and significant, the same rule should still stand. All the co-creating you could ever do, your mother's death wouldn't have happened without her choosing that road for herself.
How often do you have these guilty thoughts? You have to start monitoring where your mind is leading you and start setting it on a better track.