What i would suggets doing is maybe just tyring to relax and remember that shes not perfect, your cousins not perfect, and your not perfect, and its ok that you have problems with new people because we all have different weaknesses, Just go downstairs and try to be friendly, and try not to stress yourself out, Just be polite and have simple small talk conversations, and after a while of doing that youll begin to feel more at ease, ithats what i do when im around new people, bei polite and start with simple small talk conversations, maybe a simple, oh man its so hot out im sweating, anything like that to get a simple small talk conversation going, Just remember that, go ahead dwownstairs, and be with your family and the ones you love, and just remember no ones perfect, and if they have a problem with you its their own fault because im sure if they looked deep down inside of themselves theyd see no one is perfect, and you accept them for their imperfections and they should do the same for you no matter what 
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It is not gods dream that carries out our duties, it is our duties that carry out gods dream