Amy Robinson, 27, was arrested Friday, more than a month after the blaze in North Charleston that killed a 3-month-old girl and a 2-year-old boy. A 4-year-old girl escaped without injury.
Immediately after the fire, Robinson told reporters a knocked-over lamp started the blaze. But investigators say the children were playing with a cigarette lighter and Robinson was not home.
Police say they were suspicious because Robinson had no scratches or burns.
Robinson is in jail awaiting a bond hearing. It wasn't immediately clear if she had an attorney.