Yes, AIIMS will accept both your NIOS improvement marksheet and your original ISC marksheet. There's no issue in having two marksheets from different boards. In fact, it's quite common for students to improve their scores through NIOS or other boards.

AIIMS will evaluate both documents during the admission process. Usually, the best scores from either board will be taken into account for admission purposes. AIIMS treats both NIOS and ISC marksheets equally.

I hope this information helps you.