Sirius's friends, like the Marauders, would be extremely shocked. They would probably be very concerned about Sirius as they know his feelings towards Bellatrix as an enemy. They might try to convince him to break away from the situation, thinking it's some sort of dark plot against him. James, in particular, would be very protective and would want to figure out what's really going on.
A possible plot could be that they are under the influence of a powerful mind - controlling entity. In this fanfiction, the entity forces them into a relationship and pregnancy. The story could then revolve around how they break free from the control and deal with the consequences of the pregnancy. It could explore themes of free will, redemption, and the power of love against dark forces.
There are some out there. However, it's important to note that Sirius and Bellatrix come from different sides in the 'Harry Potter' universe, so these fanfictions often involve complex character developments and reinterpretations. You might have to sift through a lot to find the really well - written ones.
A lot of these fanfictions tend to explore the idea of redemption or corruption. For instance, Bellatrix might be portrayed as being 'corrupted' by her dark side, and Sirius could be seen as a potential 'redeemer' in some stories. And of course, the sexual tension and chemistry between them is a big part of the lemon aspect, often explored through their encounters. This can be a very complex and often controversial exploration in fanfiction writing.
There's also 'Twisted Fates'. This fanfiction plays with the idea of fate and how Sirius Black and Bellatrix Lestrange could have had a different kind of connection. It's a very creative piece that takes liberties with the characters but still stays true to their core traits. The story is full of unexpected twists and turns that keep you hooked from start to finish.
When writing a Bellatrix pregnant fanfiction, consider the setting. If it's during the height of the war, her pregnancy would be a big risk. She would have to worry about the safety of her unborn child. You could also include other characters' reactions. For example, how Draco might react if he found out, or what Narcissa would think. Build the story around these interactions and the internal struggle Bellatrix would face.
In a fanfiction, Bellatrix's pregnancy could lead to a change of heart. She might start to question the Dark Lord's ways when she thinks about the future of her unborn child. For example, she could consider leaving the Death Eaters to protect her baby. This would create a lot of internal conflict for her, as she has been so loyal to Voldemort for so long.
There are many fanfiction platforms where you might find Sirius Bellatrix stories, like Fanfiction.net or Archive of Our Own. However, I can't specifically recommend a 'lemon' (which usually implies sexual content) fanfiction as it goes against the rules of many platforms. You can search for non - explicit stories with their complex relationship as the focus on these sites.
Another option is Wattpad. Although it has a more diverse range of content, you can still come across some interesting Sirius and Bellatrix love fanfictions. It might take a bit more searching compared to AO3, but it's worth a look.
One possible storyline could be that Bellatrix, despite her dark past, has a change of heart due to some magical influence or a deep - seated love for the Potter family. Sirius, of course, would be a positive influence. They might raise Harry in a more unorthodox way, teaching him both light and dark magic in a balanced way so he can understand the full spectrum. This could lead to Harry being a more powerful and empathetic wizard, able to bridge the gap between different magical factions.