The Binding of Isaac: Rebirth

To unlock the twins, you have to defeat Mother once. This guide assumes you have the following cheevo:

which is earned for defeating Hush 3 times, which unlocks Downpour.

Defeating Mother is difficult, but getting to her is also a process. It will take some planning right from the beginning of your run. On the first or second floor, save a key to open the blue door after defeating the floor boss. The blue door leads to a new environment, Downpour. On the second floor of Downpour, you'll need to find 3 rooms: one with a white fireplace, one with a mirror on the wall, and the treasure room. Once you have found all 3, touch the white fireplace to turn into the Lost. Then get to the mirror and enter it. In the mirror world you need to get to the treasure room, be careful on the way because some rooms might have repopulated with enemies. Grab the knife piece in the treasure room and then return through the mirror.

After you defeat the boss on this floor, you'll need 2 bombs to open the door to the Mines. On the second floor of the mines, you need to look for 3 yellow buttons on the ground. They will be in 3 different rooms (one of them is the minecart room). Once you have pushed all 3, hop in the minecart and it will take you north to a secret area. Go all the way through these empty rooms, taking note of the path as you go. At the end, grab the second piece of the knife. Mother's Shadow will appear, she's a pain in the ass. She will chase you all the way back to the minecart room. She is invincible so don't try to hurt her, just run. Every few seconds she will charge you, this is easily dodged but the path will get in your way. You'll have to use bomb grimaces and quick reflexes to get out of here. If you can fly, either with Azazel or the Lost, this will be much easier (I forgot to mention you will lose all your items when you ride the minecart, so if you have flight from an item, you lose it temporarily during the Mother's Shadow portion).

Great job so far! We're still not done though. After defeating the floor boss here in the Mines, you'll see a new door with a sharp thingy sticking out of it. You'll have to poke yourself with it twice, inflicting two full hearts of damage (the Lost can do this for free), to open it and enter Mausoleum, another new environment. Mausoleum is difficult and you'll have to make it to the second floor, where a more powerful version of Mom awaits you. Defeat her, then use your specially crafted knife to open the flesh door that appears afterwards (this will destroy the knife). Go through that door, defeat the upgraded Mom's Heart, then jump through the trapdoor into Corpse.

We're almost there. Get through both floors of Corpse and you'll find Mother waiting for you. She has two phases and fires lots of tears, so hopefully you have some way to tank, or otherwise you have so much damage that you kill her quickly. Once you have defeated Mother for the first time, you will unlock a bunch of stuff, including the twins, and the run ends. Congrats!