You know, for someone that's studying medicine, you sure do know very little about "the contrast effect."
I strongly recommend: link
I remember when my womanizer uncle took me with him one day as "bait" (i.e., to have something to attract women in their late 20's and early 30's to our group). Needless to say, the plan backfired. He had absolutely zero success with anyone the whole night. Women were confronted with a choice: (1) young, good-looking, non-sleezy, up-and-coming lawyer and accountant, or (2) older, average-looking, superficially charming and sexually eager MBA executive. Picking (1) would be fun, whereas picking (2) "in that situation" would make them feel like a whore, a regular street prostitute simply electing to sell herself to the highest bidder - a negative feeling each sought to avoid. And, since I didn't share his interest in casual sex for sport, we all walked away unhinged that evening.
Next to your cousin, you're "the short one," the "less attractive one," and the "less hot one." You work wonders for your cousin, however. You are the short yard-stick women unconsciously measure him against, an immediate reference point you provide for him to shine.
For starters, you should try to formulate a social group of a few men, maybe one or two, who are also around 5'8", also about physically average, and just a tad bit less attractive than you. Why? Because "the group" as a whole must still be "worth" associating with.
Now, in "that" group, you are the alpha male. You are the "better" looking one, and you have the "highest" expected future earnings potential. You are the male with the higher "relative" social status and power. Thus, given the choice, or rather, presented with the choice of socializing on a deeper level with any "one" male in the group of males, females will "prefer" to socialize with YOU.
Now, the bigger issue is your obsession with keeping "score."
You're still a virgin, so I don't blame you as a man. It's very important to a man's psychology to get rid of his virginity. But, after that happens, you'll realize that sex is really not that amazing. Sex is actually pretty crappy unless it's with the right person. As a physical activity which rewards you with the release of pleasure hormones, masturbation is superior if you do a cost/benefit analysis. Yet, as an "emotional" activity, sex "with the right person" (because of the relationship the two of you have built together) is far more rewarding. To get there though, you obviously need to have sexual experience "together," and build sexual comfort. But, you'll find yourself empty, alone, and even more unfulfilled if your sole or primary aspiration is to keep "score" with your cousin.