I don't know if you still need information but I have had this in the past and my son had it last month, they treat you with a combination of 2 antibiotics, normally oxygen and also asthma nebuliser if the airway is restricted. It is serious but very treatable. Anyone can get pneumonia, my dad is a frequent sufferer as he also has COPD a lung condition which causes weakness in that area. if caught early pneumonia does not need hospitalisation. It sounds though as if your BIL has been unfortunate in this case. The treatment should pretty much be via drip into vein but he may go home with medication and when he does leave hospital he may well feel weak for a couple of weeks. Hope this helped. If theres anything else I may be able to answer let me know. Hope he's better asap and hope your sister looks after herself!
All the best