Is it a sin for a brother (16 years old) and his elder sister (18 years old) to sit near to each other? Is there a hadith regarding this?
No it is not a sin, as they are not suitable to marry each other.
Here is a quote from the Quran indicating that women do not have to cover themselves around their brothers:
And tell the believing women to reduce [some] of their vision and guard their private parts and not expose their adornment except that which [necessarily] appears thereof and to wrap [a portion of] their headcovers over their chests and not expose their adornment except to their husbands, their fathers, their husbands' fathers, their sons, their husbands' sons, their brothers, their brothers' sons, their sisters' sons, their women, that which their right hands possess, or those male attendants having no physical desire, or children who are not yet aware of the private aspects of women. And let them not stamp their feet to make known what they conceal of their adornment. And turn to Allah in repentance, all of you, O believers, that you might succeed.
Surat An-Nūr 24:31