Answer
The answer is half-brother. Peter I and Ivan V were the sons of Tsar Alexei Mikhailovich, but they had different mothers. Peter's mother was Tsaritsa Maria Ilyinichna Miloslavskaya, while Ivan's mother was Tsaritsa Natalya Kirillovna Naryshkina. Therefore, Peter and Ivan were half-brothers, sharing the same father but not the same mother.