Police in Lethbridge have charged a 12-year-old boy and say he sexually assaulted his two younger sisters.

Lethbridge Regional Police received a tip last month about a pre-teen boy engaging in sexual activity with his 10-year-old and eight-year-old sisters.

Police launched an investigation and found that the boy sexually assaulted both girls on multiple occasions between May and July 2014.

The boy, who cannot be identified, is charged with two counts each of incest, sexual assault, sexual interference and invitation to sexual touching.

The boy was released from custody with conditions that he not be in the presence of anyone under 12 unless supervised by a responsible adult.

He is also required to follow all house rules set out by his parents.

He made his first appearance in youth court last week and the case has been adjourned until August 27.