Honestly I do actually see Oda doing this, I've had suspicions of it happening since we got spoilers for last chapter and I'm I'm not the only one thinking it could be a possibility:
https://www.reddit.com/r/OnePieceSpoilers/comments/1n4mfvy
Would be a unique tragedy that really adds depth to Shanks and create a real schism between him and Shamrock as individuals/characters based on their beliefs/personalities or maybe they both secretly seek revenge on Garling too? I can see Garling being sadistic/sick enough to kill off their mother
and it would be a perfect way to cover up that he had kids with a slave/civilian or w/e too.
I would really root for Shanks to get revenge and kill Garling if this was true too.
Oda loves killing off so many mothers after all. Waiting for the inevitable Nami's mother death reveal.
Somehow this will all tie into the mother flame (not to be confused with mother frame) too /s.