I2C extender or repeater?
Where did you get the idea that SDA and SCL lines should be twisted?

This is the second worst thing you can do to I2C communication next to cutting the wires.

Twisting is suitable for differential lines such as CAN or RS485/422.

With slow enough communication you probably won't even need repeater/extender on 2.5m. Especially, if this a 5V system. Just untwist those I2C lines.

If you have some spare lines on the ribbon cable try to put some GND lines in between I2C (non-twisted )pairs or even between SDA and SCL of same channel.

EDIT: there is a way where twisted-pair might come in handy for I2C. It is where each of the two signals is transmitted differentialy (some kind of 4-wire I2C). 

Driving I2C-bus signals over twisted pair cables with PCA9605
transmits SDA bidirectionally over a twisted pair of wires.

Sending I2C-bus signals via long communications cables with P82B96 or PCA9600
explains the advantages of transmitting SDA using a unidirectional "4-wire driving method".