Can lice cause sepsis infection?

Not directly. However, if the lice infestation is severe enough that the patient lacerates her head it can lead to skin infection (cellulitis) and this can progress to sepsis, shock, and death. Lice themselves, however, don't cause sepsis.
Rarely. Very old, very debilitated patients, and people with advanced disease (such as metastatic cancer) that has ravaged their immune system, could conceivably become septic if a bad lice infestation is left untreated long enough. But for anyone with a relatively intact immune system, such a complication would be extraordinarily unusual.