Europe is a leader in sustainable investing which has coincided with an increase in the number of exchange-traded funds (ETFs), offering sustainability exposure. However, many of these products are structured synthetically using total return swaps, and due to a lack of clear regulatory guidelines, synthetically structured ETFs that track sustainable indices have been found to hold non-sustainable assets in the collateral baskets, which form part of the structure of these swaps. This underscores the need for enhanced oversight concerning fiduciary responsibilities to investors. We propose an augmented role for index providers in stewardship activities, leveraging their substantial influence within the industry. Furthermore, we highlight the potential impact of sustainability criteria on collateral baskets, which could significantly boost flows into sustainable finance.