This LW reminds me of myself and my sisters. I've always struggled to maintain a healthy not too thin weight and they both struggle to keep enough weight off to not get the run around from doctors. We've all had these issues since roughly puberty hit. Turns out our thyroids are all fucked. Mom's too.

I'm not saying that unhealthy snacking habits or medication may be playing a part, but you should get some bloodwork done too specifically checking out hormone levels that can fuck with weight gain in either direction. Getting each girl a solo visit with a nutritionist to help with a food plan may be a good idea too.