No rank downs in ranked

So I was playing ranked with my brother and he was, and still is, silver 5 after losing 8 matches solo and two ranked with me, which he also lost. He had 0LP at the time and he didn't rank down. I was also Silver 3 with 0 LP and lost two games, thanks to him, and I didn't rank down. How come? Also, he has been doing this weird strategy lately where he "sacrifices" himself so we can get objectives but that put him on like a 10 loss streak and he still blames the team even though he is the one feeding. When I try explaining to him that maybe he should try a different strategy he gets extremely diffensive. So how come we didn't rank down? I know that a person usually wouldn't complain about ranking down but if theirs an issue it should be addressed.