Is it wrong to wear your sister's thongs if you are a boy?

Given the use of 'thongs' for different items of clothing, maybe. If you refer to jandals (or flip-flops, or whatever else you may call them) it is probably not good to wear them as they may not be the right size for you and cross-infection for things like athlete's foot or other fungi may happen. It is easy enough to get your own and these could be in a better style and fit for you.

If by 'thong' you mean a G-string or similar piece of underwear, it is probably considered wrong to wear those belonging to your sister. However, at a young age many people like to experiment, as it were. You may wish to talk to your parents about this and ask them about it; you will not be the first person to have done something similar and they can talk to you about exploring that avenue of your personality further.

well technically it is wrong but a lot of boys want to find out what it is like to wear one