I killed Grelod the Kind and completed the quest by talking to that boy. He gave me a reward and the quest Innocence Lost was finished.

After that, my character tried to sleep all the time for several in game months but she is never kidnapped. A courier did show up once but it was only regarding something from a Jarl. Also, I'm now hearing rumours about a bride in Solitude planning her wedding. Is it too late to join the Dark Brotherhood now?

I tried console commands but Astrid only says Sister and can't progress any further so I reverted back to before I put the console command. Any help would be highly appreciated.

Do you have any mods installed

Yes. I have the following mods installed: Skywind, Skyblivion, More Interesting NPCs, all of the unofficial patches, better quest descriptions, spells of the third era, third era birthsigns and classes and third era attributes.