Can u help me I feel like I'm losing my family?

My 6 year old step son has ruined my families life by verbally abusing and physically abusing my 5 year old daughter. Since she came to me 4 weeks ago telling me how he violently hurt her girl parts by forcing his self on her after telling him no, he proceeded to kick, knee, and punch her in the girl parts and bullies her. Ive kept them apart in fear for her and my 3 year olds safety. It's been horrible :( I cry all the time. My kids cry Bc they can't be here when he is. Thank god it's only everyother weekend! I've talked to my husband to get him help and in turn he tells his sons mom to take care of this but nothings being done. I'm sooo depressed and stressed and I love my husband and I feel like his intentions are good he's just not fixing the problem and ontop of everything my husband wants sex and critizes me for not wanting to have sex! I'm just carrying this heartbroken weight on me I can't think of anything else. Help