Once you fail to board any flight on the itinerary, any later flights will be canceled. So in your situation, when you do not board the IND-ORD flight, your ORD-SEA flight will be automatically canceled.
It's possible that if you do skip the first flight and simply try and board at Chicago that you will be able to talk them into re-instating your booking, but the only way you'll be able to do that is to lie to them and claim you did fly the IND-ORD leg and hope that they believe you, but keep in mind if you do this that :
- You will be lying, and possibly committing fraud.
- There's a good chance they will not believe you, and ORD-SEA is a long way to walk!
- You will not be able to check any bags
- You will need to check-in online, at least an hour before the IND-ORD flight departs
The only safe option is to change your ticket. Depending on your status with United, this will cost you $75 (non-elite), $50 (Premier Silver), $25 (Premier Gold) or nothing (Premier 1K).