By Tom Sims and Carolyn Cohn FRANKFURT/LONDON (Reuters) -European bank shares started to rebound on Friday after steep falls a day earlier, as bankers grapple with the potential impact of a slew of sanctions following Russia’s invasion of Ukraine. Shares wavered as missiles pounded the Ukrainian capital and its President Volodymyr Zelenskiy pleaded with the international community to do more, saying sanctions announced so far were not enough. After early losses, leading European banks later edged up, with a European banking sector index 3.7% higher by mid-afternoon, only partly making up for T…