I don't think there is any way to judge your own appearance objectively. I think guys just don't want to come across as creepers online. You show someone who doesn't know you a picture of you and your sister and ask which one is prettier (if you truly want to know)
I doubt most people can be truly objective, they're either too harsh or have an inflated opinion of their attractiveness. The best indicator is external observations. If you've been told most your life you are attractive and are told consistently and frequently, it's a good bet you are. If you are told occasionally then you're most likely above average. If you get few compliments you're probably average attractiveness. And finally if you've been teased or called ugly throughout your life, then, sadly, that's probably the case.
You get specifc, and lots of compliments on fb. Or you post a picture and ask people for their honest answer. Most girls I see look pretty decent, obesity would be the main reason for being unattractive imo.
If you attract the attention of the opposite gender a lot in real life then you are likely physically attractive. If you attract the attention of the opposite gender online, then you likely have an attractive personality.
Well everyone has a different idea of attractive but if you got compliments often or were approached I think those would be indicators. But at the same time there are very attractive people who aren't approached so often because of their personality (maybe they aren't as outgoing) so its hard to say. I think any physical attractiveness is irrelevant if someone is ugly inside. Maybe guys are more receptive in real life because they can visually see what is real, as online anyone can say they are anyone...
I don't know about the sister thing... I personally wouldn't care about something like that.
Every1 has different ideas to being attractive there's no right or wrong answer one person might find you attractive another person might not it's up to you to decide weather you find you're looks attractive or not even if you don't think you are some other people will depends what sort of pics do you have up slightly racy ones or do you go to extra effort to do you're self up in you're pics some guys are really shy irl it's a lot easier for them to talk to you on line then irl also it's a matter of opinion if you're prettier then you're sister some people might think so others might not it's up to you to decide weather you're prettier then you're sister.