Varna,
Bonding can be very difficult and frustrating, or it can be love at first site. It doesnt matter if two rabbits are from the same litter, they may still fight. Just bringing home two rabbits from the same litter and hoping for them to be forever friends will not always work. Often, two rabbits who are best buds when they are babies will fight as they mature.
With regards to sex, a male and a female are considered the easiest to bond, then two females, and then two males. That being said, it depends more on the personality and less on the sex of the rabbit. There is also no guarentee that by bringing homea male and a female that they will be friends.
It is important that both rabbits be altered before introductions, to ensure there arent any unwanted pregnancies and to help reduce aggression. Rabbits who are neutered tend to be less territorial, which helps when bonding.
I hope this answers your question!
-Haley