PSG host relegation battling Le Havre on Saturday
Pique headed the Catalans in front in the 36th minute, connecting with a cross from the lively Ousmane Dembele who produced another decisive display after scoring Barca's late equaliser against Atletico Madrid last week.
Homegrown midfielder Alena, 20, came off the bench to score his first league goal for Barca in the 87th minute and seal the win, racing on to a through ball from Lionel Messi and producing a confident finish.
Barca lead the standings with 28 points after 14 games, while Sevilla are second on 26 and can re-take top spot if they beat Alaves later on Sunday.