Ma Santoshi is daughter of Lord Ganesha. Ganesha has two wives Ridhi and Sidhi and two sons Shubh and Labh. 

Goddess Mansa with Lord Ganesha on Rakshabandhan

On the day of Rakshabandhan when Mansa Devi tied Rakhi to her brother Ganesha , then Shubh and Labh asked their Mothers about Rakhi and the festival. Both the sons Subh and Labh asked their father to give them a sister who will tie them rakhi.

Subh and Labh asking about Rakhshabandhan

Subh labh asked their father to give them a sister as they also want to celebrate Rakhi.

Ganesha decided to give them sister and he from his powers gave them their wish, Santoshi Ma .

Subh Labh asking  Ganesha to give them a Sister

Santoshi mata tied rakhi to her brothers Subh and Labh.

Santoshi Ma tied Rakhi to Subh and Labh

Santoshi ma is the goddess of Satisfaction .