Fog cloud does not end sanctuary
If the warded creature makes an attack, casts a spell that affects an enemy, or deals damage to another creature, this spell ends.
Fog could states:
You create a 20-foot-radius sphere of fog centered on a point within range. The sphere spreads around corners, and its area is heavily obscured. It lasts for the duration or until a wind of moderate or greater speed (at least 10 miles per hour) disperses it.
At no point does fog cloud affect a creature, it only affects the area which becomes heavily obscured. Hence, fog cloud does not end sanctuary.
A heavily obscured area states:
A heavily obscured area--such as darkness, opaque fog, or dense foliage--blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.
However, the blinded condition is not an effect of fog cloud, it's the normal interaction between a creature and the environment.