I think attraction is important. However I also recognise that the men I find attractive are different to the men my female relations find attractive. I don't like sporty, traditionally good looking types, with fancy cars who are working their way up the slippery pole in the office. I'm sure some are lovely people but they have never done anything for me, nothing in common plus no physical attraction. Despite many people saying how gorgeous etc such men are. I know a few women who have married the traditional football/rugby/career types, they moan about sports/work taking over, no interest in kids, going out with the boys, no romantic gestures etc. This would bore me, so I never dated one.
Could it be that your sister gets the feeling you are embarrassed by your boyfriend or not holding hands etc and thinks you don't like him. However you are acting this way because you are worried by what your family will say!
Try not to look at him negatively, it's easy to find someone unattractive when you look for physical flaws and focus on them. Does he have nice eyes? A great smile? Focus on the good bits, remind yourself of these good bits.
Introduce them after speaking to them about this fantastic guy you know. You love the fact he is into x, y and z. You have been to x place recently and you enjoyed whatever about it. Don't mention how he looks, just tell them he is great and you are pleased you have met a guy who you have so much fun with.
Your sisters response when she meets him will tell you a lot. His looks shouldn't come up.