Brothers do this type of shit for a number of reasons and none of them are either particularly well thought-out nor evil. He loves you and wants to you to get better, but he also maybe a little insecure about losing his spot in the food chain, and generally he wants to rag on you because hey, you're his little bro and that's just how it works. It's totally natural and it's called "being brothers". (Source: am big brother)

That said, if you want to really get this to change, you should do a couple of things. Someone has said 'talk to him about it', and if you have that sort of relationship, that's the best way of course. However, if you're like most brothers I've known, this isn't a real option.

First, you should simply internalize the problem and make it not an issue for you. You can't fake not being bothered by it. He picks up on your reaction like seeing a lion seeing a limping gazelle. Don't let the relationship amplify the effect; BJJ is going to mean a lifetime of tapping to people and that your brother is just another. 50lbs is not a trivial difference either BTW. I've been doing this for a long time and when I roll with someone near my level and they have 50 solid pounds on me, I know it's going to be some work. Simply put, agree with him or even ask him for help: "you're totally right" "you got me good on that one" "could you show me how that technique is supposed to work?" "good to roll with you man" and "you're so tough". Ask him questions, "man, when you mount, I just cannot get you off of me...do you have any tips for how I can do better?" These types of responses reassure him that he's still the 'big brother' and even put you into a position where you're asking him to help you beat him. If you work that angle right, you -might- eventually convince him that he should be taking you under his wing, not mauling you every chance he gets. I give you one chance in two on that though...you're brothers after all.

Secondly, optionally, keep that ember burning in the back of your mind and train. Hard. Train mount escapes like all damn day. Keep training until you can mount -him- and then give him a wet willy and call him a sissy on the way home. This is how this brothers shit works.