Both players arrived from Portuguese clubs, with Lopes, 24, signing from Nacional and 20-year-old Sissoko joining from Academica de Coimbra.
“Lopes is tall and quick, which is everything you need in a central defender,” Wolfsburg coach Felix Magath told the club website.
Magath described Sissoko as “a young player for the future blessed with good technique.”
The new arrivals take Wolfsburg’s January recruits to seven, after Vieirinha (PAOK Salonika), Giovanni Sio (FC Sion), Slobodan Medojevic (Vojvodina Novi Sad), Ferhan Hasani (Skendija Tetovo) and Petr Jiracek (Viktoria Pilsen).
Wolfsburg, the 2009 Bundesliga champions, had announced the signings of Vieirinha and Sio on Wednesday.