please help

i have a 17 month old daughter from a previous relationship and my partner has a 4 year old, also from a previous relationship. i met him when my little one was about 9/10 months old and he took her in as his own from the moment he met her. we have a healthy and happy relationship and love our little blended family. although, we have been heavily struggling with his 4 year olds behavior. we believe there is something going on at her moms house yet she won’t give us any insight. (she gets extremely angry anytime we ask) and we are in the process of getting full custody. but PLEASE does anybody have any ideas on how to navigate her behaviors? they are so intense and extreme ALL the time. she hits, scratches, and bites herself. constant screaming and whining. and when i say all the time i truly mean ALL the time. from the moment she wakes up until she goes to sleep. she will also hit us. her mother also has a 4 month old who she has hit as well. she also has essentially attacked my toddler. she pulled her hair, hit her, and attempted to dig her fingernails into her forehead. she’s extremely possessive and will taunt/bait my daughter with a toy and then get angry when she attempts to reach for her and then try to get us to discipline my toddler for it. and no matter how we try to discipline or redirect her, it NEVER works. we live on a top floor apartment so when we do time outs or try to disengage because she’s being aggressive, she will stomp on the floor. she will also break stuff, throw things, slam toys against the wall, etc. we got her into occupational therapy and talk/play therapy once every week but we are still very much struggling with what to do in the meantime. if anybody has had any experience with this, it would be greatly appreciated to know what helped or that we’re simply not alone. sorry for the long post! but thank you if you made it this far!