It will help always to think of your storylines as a team of unruly horses that you have to drive. Each will always be trying to pull you off course, which, if it happens, will mean the whole project will go off course - and you the writer may be so distracted and so interested in that one horse that you won’t even notice the whole project heading straight for a ditch.
This happens to the best of writers because this stuff is just so difficult, so be prepared for it to happen and keep double checking.
Writers creating multiple storyline projects on their own (films,TV pieces or games) are articularly prone to going off course because they are one writer running a whole massive project on their own, with nobody to give an objective second view (this is one major reason why TV shows have so many people monitoring scripts as they progress).
White boards for each storyline will help you to differentiate and control. Create stories separately, then interweave, pruning back storylines to fit Think in terms of ‘story beats’ and use the old fashioned tried but true TV story-beat estimate for each ‘episode’ to give you somewhere to start in planning how much story you need. The storybeat principle is that one creates A, B, and C stories. You will instantly recognise this from the TV series you have seen. A is the main story (in your case the gang warfare story, linking the gangs) B is the serial element (in your case ongoing fights/love etc within families) C is a short story complete in that episode. For fifty minutes of TV we used to calculate 36 beats: 18 A plot, 12 B plot, 6 C plot.
Just stick to that ratio 3:2:1. Index cards are very good idea here, one card per beat. This is just a start. Depending on your material, may want to give more to the B story, or split the B story up into a number of smaller stories. Important point. Plot A will present itself with a timeline. Peg the other stories to that.
Remember, a beat is a step in the story, not a scene, and you can combine two or beats in one scene. You may have several scenes to a beat. For more on all of this, see my TV ebook Television Writing: The Ground Rules of Series, Serial and Sitcom and my book on screenwriting, The 21st Century Screenplay pp 127-164 on practical plotting and the chapters on Multiple Protagonist structuree pp. 207-245 (by the way, for people outside of N. America, Google Play books is currently running a specialbargain deal on the ebook of The 21st Century Screenplay).