Accept who/what you are attracted to, as long as what you are interested is another consenting adult, then who cares what your sister or anyone else thinks.

And try looking at things from the other side, an overweight man who has lived his whole life being judged and looked down on due to his weight, think how wonderful it would be for them to find someone who would love and accept them just how they are. For get your sister and those like her, go out and bring true joy to someone who probably is in desperate need of it.