Assalamu Alaikum! can a brother and his sister sleep on the same bed? Someone has recently told me that it's haram.
Alaikumassalam wa rahmatullah,
There is one hadith in Abu Dawud and the Musnad in which the Prophet 
says brothers and sisters should not sleep in the same bed after the age of ten:
The Messenger of Allah (ﷺ) said: Command your children to pray when they become seven years old, and beat them for it (prayer) when they become ten years old; and arrange their beds (to sleep) separately. (Sunan Abi Dawud 495)
This hadith, however, is very low-quality and Imam Abu Dawud himself says the hadiths of this transmitter (Amr b. Shuayb) are not to be used as evidence for legal rulings when he transmits from his grandfather through his father (like in this hadith). So the common Muslim belief that the Prophet recommended beating 10-year-old children if they do not pray is based on very weak evidence that Islamic law rejects. The same applies to the command to have them sleep separately. But it is common sense not to let adolescent boys and girls sleep together even if they are siblings.