"Lets say there's 100mA available for a port pin"

What do you mean by "available"?

"the pin can sink(yay!) 26mA. What will happen?"

You will need to read the datasheet to find out precisely what that "26mA" actually means.

Normally it means, "if you put more than 26mA through it, it is not guaranteed to survive unharmed"

I've said this before: It might just happen to survive on the particular sample that you try it on, or you might not notice the degradation in your tests, but this is no basis for a product design - you have no guarantee that it will always work properly under all conditions for the whole product lifetime.

You must always design within the datasheet limits if you want to guarantee that your product will work.