Hey Kennedy, it depends how new the stretch marks are? If they are recent they'll be purple/red and fairly hard to get rid of at this stage. Eventually they will fade to a light silver/white color so are way less noticaeble (ive got some on my hips but only see them in certain light
You could try doing some resistance exercise for this area, as this will strengethen the muscles underneath, making the skin firmer and making them less noticeable - this is perfect for cellulite too. I mean quite heavy weights for squats, lunges etc depending where the marks are - and it's not a quick fix but will work AND prevent new ones from forming (they happen when we get sudden weight gain and the skin cant keep up with the change and 'tears' causing the mark, which is like a scar...)
i can tell you that every girl i know has some, and working as a beauty therapist ive seen a lot of bodies and can tell you even some guys have them.
you sister probably points them out to make herself feel better - remember that!! sisters are terrible sometimes...
quick fixes? keep the area moisturized with cocoa butter/coconut oil or any other heavy cream to ensure they don't get worse.
there are treatments you can get in spas, but they require a course of about 6weeks, are expensive, and am not sure they're worth it.
also try lying on your back with your legs up frequently, massage your legs from feet downwards to help the blood drain, and help ensure the skin is nourished and clear. this also prevents varicose veins and tones the skin.
i hope ive been of some help, remember your sister is being a pain and no one else actually cares about a few stretch marks on your thighs if they ever actually even see them! they will fade, so look after your skin now, wear 15denier pantyhose if you want to cover up (they make your skin look amazing), and have fun
xxx