With the inflation and economy as it is is a perfectly valid and reasonable reply is to say “Sorry I cannot do it for free anymore. I have to charge you X”. Because everything costs more now than it has. She cannot expect free photos every year, that is using you. Be firm and strict, you can say NO to family and is an acceptable answer.
It comes with time and practice. Once you figure out your niche and style you look for the light and gravitate towards it. But it’s good to be diversified and skilled in all lighting scenarios.
I’ve been in the photography world professionally for a long time (13 years) and I know when and where good light is, I can look out a window of a car and see it, but this changes with seasons, weather, location, other factors. Since I professionally photograph weddings and portraits I tend to look for shade but not too much. My favorite is trees with light coming from behind the subject. Things like where your subject is standing can include the colors related back onto them. But if light isn’t preferable I know what to do to make up for it as much as possible.
You can’t have perfect light every time you go out and shoot. Sometimes your eye is caught by something and you try it out. Photography is about experimenting!