This question, Why are my photos not crisp?, covers a multitude of possible reasons why images may not be as sharp as we might want them to be. How do I diagnose the source of focus problem in a camera? addresses how one might determine what the specific reason for a particular case is. This answer discusses ways to improve getting sharp results on the intended subject when shooting at large apertures.

In the case of the example photo, the image is slightly front focused. Notice that the hands of the boy and the girl are sharper than their faces. This could be due to user error in using the camera's AF system, or it might be due to a camera and lens combination where the manufacturing tolerances of each work to pull the focus forward of where the AF system is instructing the lens to focus.

Putting the camera on a stable tripod and triggering the shutter via timer or remote (wired cable release or wireless) will probably result in an even sharper image, but it will do nothing for a front or back focus issue. For that the camera and lens need to be calibrated to each other. Since the 77D does not offer Autofocus Micro Adjust, that would require a trip to a Canon Service Center to adjust the camera to the lens or the use of the Sigma USB dock to adjust the lens to the camera.

I do focus and recompose a lot.

Focus and recompose is susceptible to missed focus when using wide apertures for two reasons:

• The camera can be moved closer/further from the subject as you recompose. Ideally instead of rotating the camera around the center of the photographer's body - which is what usually happens - you should rotate the camera around the optical center of the lens (the so-called 'nodal point')
• Even when rotating the camera around the optical center, depending on the shape of the lens' field of focus, you can move the subject in front of or behind the field of focus