Longtime rivalry with older cousin

My older cousin and I have always kind of resented each other. My late grandmother favoured me while her husband, my grandfather, favoured her. When my grandmother was alive she resented all the time I spent with my grandmother and the things my grandmother and I would do together such as play board games, cook together, or put up holiday decorations. But when my grandmother died, I had to spend more time with my grandfather and it was obvious my older cousin was his favourite as well as everybody else's. My grandfather and I never liked each other, although we are cordial now, and my cousin is still the family favourite. It seems like no matter what I do, I cannot one up her. For example, eighteen months ago I started working out, and I lost forty pounds. I had to stop because I ran out of time and motivation, unfortunately today we had a family function and my older cousin walks in even skinnier than I am. That hurt, especially since I show signs of narcissistic personality disorder (and plan to get evaluations and treatment) and hate being shown up. I cannot confirm she was taking a leaf out of my book, she is in college and perhaps cannot afford to eat as much or something. Even worse? An hour later after I had to endure everybody telling my cousin how beautiful she looks now and greeting her and making conversation, she's all over the two littlest cousins, doing that silly baby voice, playing peekaboo, and chasing them around. I am HOPELESS with children - I'm not happy and playful and bright. I don't care to sing to them and jump them around, I can't even "superman" my three-year-old second cousin very well while she was playing incredibly well with that cousin, and to me family functions are basically babysitting competitions/baby fests now that a few new Gen Alpha babies have turned up.

Everybody absolutely adores my cousin and her interactive personality and how well she is with babies and how talkative and outgoing she is, and they always want to see her. But me, I'm just the wallflower cousin. When I was very young I used to cause problems at family functions because they would be overstimulating (I am autistic), and because I never liked my grandfather, but now that I can deal with it I'm sure I can't shake the label of the aloof, rude cousin. I can't even get my own family dog to like me - I'm so bland and shy. It makes me feel absolutely worthless (and I still have a while to go before I can escape and go away to university).

How can I get my extended family to interact with me and always want to see me the way they do my cousin? How can I be more talkative and fun and how can I be better with the babies when I am so naturally inept with them and bad at playing toddler-friendly games, doing a happy baby voice, and running around? Am I being overdramatic here? Am I reading into it wrong and holding onto an outdated perception of myself long after my family shook it off, or feeding into it by retreating because I don't think they want to spend time with me? If I'm feeding into or holding onto the outdated perception of myself, how do I change that? My family likes it when I talk about my many activities, but rarely give me an opening to do so without stealing the conversation from somebody else. My next family function is soon - what should I do?