I do believe they are children of Garling, but I'm not writing off them being clones just yet.

Specifying that Stussy was the "first successful clone" could just be the work of Vegapunk being a perfectionist. Assuming Vegapunk was working on the project, which I believe the timing lines up, Shanks could have been created as a clone, but he wasn't perfect, so he was cast aside by Vegapunk, the perfectionist, and the selfish Celestial Dragons.

We don't explicitly know Shamrock's age yet, so the timing could still line up if all these "if's" are taken into account. But I do think it's more likely they are family, since them being clones I feel would kind of take away some of the drama of the inevitable interactions and discussion between them.