my worst week

I just found out today that my 12 year old step son has been puting his hands is my 9 year old girls panties and i did not know about it till just recently i had to go make a police report on him and this is just so hard becouse me and his father have been raising him since he was like 5 years old me and his father have been married for 10 years so now im just so numbe i feel like i want to cry but i cant this can ruin my marriage and my little girls life and i dont even know what to do next they have not investigated yet whould should i do next what is your opinion