I have the same exact thing! I got them on my knees and back when i was young and somewhat underweight, then around my early 20s i started gaining a little weight and didnt think much of it, untill about 2-3 years ago (around 23) I started gaining weight very quickly and mostly in my belly and thighs.
I am now very overweight, probably obese :( I have been to 2 doctors now and have low thyroid hormone levels, but still barely within range so they say im fine by labs and ignore everything i say and physical symptoms (huge belly with wide purple/red stretch marks, swollen hands and feet, and even swollen lymph nodes all through my neck (maybe even thyroid as well) :(
They pretty much refused to test cortisol levels saying im a young male and its too rare..
So i'm now waiting forever to see an actual endocrinologist :(
Hope you have better luck than I have so far :)
Have you asked your doctor about this? It sounds like it might be a genetic problem if your brother has them, too. I don't believe it has anything to do with the thyroid. I have not heard of this problem, yet. Maybe a skin specialist could help. Sorry I can't be of any help. Keep watching this post, though. Maybe someone will have better advice.