How will it affect other cryptocurrencies? Should I sell ETH?
You should keep your ETH since it might not affect cryptocurrencies at all. If ever the rumors about Rothschild's interest in crypto being true, I see it in a positive light because it might make people more interested in decentralized cryptocurrencies. It is also impossible for them to develop a currency that will be used by everyone around the world. IMO they just don't have enough political influence to every country for it to happen.