Behaviour help & advice please

[doubletroublex2]

My son is 3&1/2, also have 3 month old.
My son is generally very well behaved at home & nursery but when a certain family member is about its mad! His 5 year old cousin, they can be getting on the best one minute & the next he is nipping her, pulling her just, scrabing her! It's got worse. We have tried different tactics- talking, ignoring Him, attention on her, time out, time in his room. My poor niece is getting battered and bruised and I feel so guilty.
The thing is he has so much love for her at other times, they note get on great. There's no real cause- they could be in car beside each other, on swings, at times yes it's over a toy.

Secondly, he is rough around his baby brother(3months), gives him hug but pulls him as same time, nips his heAd, pulls him etc

I should say he is generally a v affectionate child.

Any words of wisdom would be much appreciated. I would contact my HV but not sure she would be much help to be honest! Are there behaviour specialist he could go to?
Thank you in advance

Have you said "NO!" in a firm voice?

Yes we have tried being very firm
Said NO

Most of the parents have several questions in mind about raising their child in the right path. Sometimes I feel like I am not doing anything to pave the way for my son to become a good human being. And, now I search for ways to help my child to overcome the challenges he would face in his life. I came through a blog explaining about emotion coaching. Emotion coaching helps the parents to guide their children to become successful adults.
It includes the following:

  Emotional awareness
  Connecting
  Listening
  Naming emotions
  Finding good solutions

Read more www.drcohen.ca/general-category/what-is-emotion-coaching/

I think it would be helpful to you.