A 41-year-old man was arrested on Tuesday for murdering his 65-year-old mother-in-law after attacking her with a wooden piece during a family dispute at Sasthamcotta in Kollam. Police on Saturday arrested Sawan Birhor for allegedly killing his wife, Manta Devi (30), and mother-in-law, Sonia Devi (50), under Tatisilwayi police station limits of the district.