Nit pick. Don't confuse between 'leader of one of the opposition parties' and 'Leader of Opposition[1]' which is an official title. Sisodia is not part of Parliament of India (let alone LoP) so the official title doesn't apply to him. He was/is(?) the excise minister of Delhi state when the 'supposed-liquor-scam' took place, hence the arrest. No idea if the allegations are legit; there are 28 states in India and one of 28 Excise minster is arrested.
[1] https://en.m.wikipedia.org/wiki/Leader_of_the_Opposition_(In...