One of my friend never had an elder nor a younger sibling and spent most of his life alone(without a sibling). He wants to have siblings. So my question is will it be possible for him in Jannah if he hopefully reaches there ?
Yes It is Indeed Possible If you get to Jannah. This is because anything you wish is possible in jannah and Allah The Most Merciful will fulfill all your wishes and desires.
In the Quranic verses, Allah the Almighty states that any desires of a person will be given in jannah.
“Therein are brought round for them trays of gold and goblets, and therein is all that souls desire and eyes find sweet. And ye are immortal therein.” (Surah al Zuhruf 43:71)
Narrated Abu Hurairah:
"The lowest place of any of you in Jannah will be that Allah will tell him to express his wish. He will wish and wish again. Allah will then ask him: Have you expressed your wish? He will answer: Yes, I have. Allah will say: `You will have what you have wished for and the like thereof along with it.''' (Muslim)
So You will Get Anything in Jannah even if someone wants an elder or younger brother sister or even a Ferrari but for that you must try your best to enter Jannah.