Sounds horrendous! I have 2 dd's, the eldest is 4, and the youngest is 2, there's 21mths between them. It must be hard with 3 children, but I would be absolutely horrified if my eldest behaved like this on a consistent basis, and it sounds like it is happening regularly. Yes, I would be asking for professional help.
good 30-40% of the weekend screaming/roaring/shouting and/or being very physically violent with her siblings. - We get screaming/roaring/shouting, but it tends to be at certain trigger points, like overtiredness at bedtime. Or when she's very anxious/insecure about something.
being put into (and running out of) time out. - Rarely need to use time out now. Maybe once in a couple of months.
She was physically locked into her room at one point because she was kicking and scratching her mother and left bruises on her arms from swinging at her/pinching her. - Wow. No. (Not yet anyway! - unless they go through some sort of personality change at 5?? Hope not.) I would be so shocked if dd1 this, as that would be completely overstepping the boundaries.
She walked up to her 3 year old sister who was playing with my son and literally, out of nowhere, caught her by the hair and banged her head on the wall. Omg - dd1 would never do this to dd2. Sure they bicker and squabble, but they also play well and care about each other. I did a lot of prep with dd1 before dd2 was born - we read "I am a Big Sister" books, talked about it, even took her to a scan appt. When dd2 arrived, in the early days there was a lot of "Look, she's watching you! She loves you and wants to be just like you!" and getting her involved in nappy changes etc. Now, we say "good sharing!" and praise co-operation type things. "That was very kind of you." etc. They still fight and bicker over things, but it's a poke here, a slap there, in the heat of arguments - nothing major, and certainly no unprovoked attacks out of the blue.
She was trying to frighten my son by going up to him and screaming in his face and then laughing when he cried. I've never seen dd1 behave like that to any child tbh, not even to her sister. If she did that to anyone's child I'd be furious and she would see it on my face. I would get down to her eye level straight away and tell her that it's NOT ok to do stuff like that, and there would certainly be time out.
My inlaws are not soft on her, they cancelled her birthday party this year because of behaviour like this and they box up access to her favourite toys when she carries on like this and she has to earn them back with good behaviour.
She missed out on lots of treats over the weekend for this behaviour but also was given lots of praise and attention for anything she did well when calm
I don't know, I wonder if your inlaws are, if anything, too punitive with taking things away, and not giving enough actual quality time or something, which could be easily missed with 3 young kids to juggle.
I don't know, I'm no expert. All I know is, if my eldest started behaving like that I would be concerned - but then she hasn't hit 5 yet, so maybe I'm in for a shock!