Procrit commented on weight loss. I will comment on strength training. With all due respect to your sister, what matters to cyclists is deadlifts, back squats, leg presses, Bulgarian split squats (one leg squats) and jumping. And strength training has to be coupled with on the bike work and periodized to support an event schedule.
I have not found that Pilates, core work and etc does much for my cycling. A back squat with an Olympic bar with weight works my legs, back and core. And this should be as heavy as possible without injuring oneself which is hard to do. And jumping is important and can be done as squat jumps, box jumps or one leg jumps over a bench and land like a cat. The landing is critical in jumping so as not to injure oneself - no slam dunking basketballs.
Core work, Pilates, yoga and all the other attending exercises are great for ones health. But it the goal is getting faster on the bike and racing then one needs leg and back strength and a core that is engaged when the legs and back are loaded. YMMV.