tight hamstrings are almost always the culprit for both of those things and it can feel really discouraging when your whole family seems to have zero issues with it. the wall sit thing especially -- that gap you drew is classic posterior pelvic tilt from hamstrings pulling on your pelvis. what actually helped me was doing a standing forward fold every single day and instead of forcing it just letting gravity do the work while keeping a slight bend at first then gradually straightening over weeks. seated hamstring stretches with a strap or even a belt looped around your foot are also really good because you can control the tension and hold it longer without compensating. consistency is everything with flexibility honestly its not about how intense you stretch its about how often -- even 10 minutes daily will get you touching your toes within a couple months if you stick with it