Here is my situation. I booked an IAH-IST round trip ticket, which has 23 kg baggage limit. Later I booked IST-NAV round trip, with the first leg IST-NAV 3 hours after my arrival in IST from IAH. This ticket was a promotional fare with 15 kg baggage limit. I called Turkish Airlines call center, and the guy told me there are 3 possibilities for the initial part of the trip.

1. The check in people in IAH will check my bag all the way to NAV with 23 kg limit.
2. The check in people in IAH will check in my bag all the way to NAV, with me paying 6 lira per kg over 15 kg because of the second leg.
3. The check in people in IAH will check my bag to IST, and in IST I will claim my bag, check in for the next flight, and pay 6 lira per kg over 15 kg.

The call center said I should go to IAH and ask the check in people what they will do. I could possibly change the flight to the next fare up which has 20 kg limit, this fare is 55 lira higher, but mathematically this doesn't make sense because even if I had 23 kg my penalty would be 48 lira which is less than 55, but I would avoid hassle at IAH or IST. Or I could just make sure I could pack less than 15 kg and avoid all the hassle altogether but I doubt my wife will be able to do that for a February trip.

Advice? And do I have to claim baggage and recheck in IST in case they check it all the way to NAV? I've never taken a connection in IST except to DEL.