TL;DR Stay professional, administration/management must handle the abusive colleagues, there's no situation where you should handle them. Have detailed documentation of everything.
Preface: IMO you would benefit from finding employment elsewhere greatly, however, I understand the hesitation to leave a company you've been with for a long time. The following advice is if you 100% intend to commit to resolve the issue internally. And remember: Always have an offer for another job before you leave your current job.
I want to take a moment to point out that companies worth their salt do everything in their power exactly to avoid these sorts of conflicts of interest. I haven't applied to a company that didn't ask me: "Do you have any family members working at [the Company] and in which departments, or otherwise is there anyone you share a relationship with who currently works here, period?"
These create conflicts of interest, and as you can see, the conflict here actually extends bottom-up quite a ways! I'm already pegging this workplace as largely unprofessional on these merits, and the resolution should obviously now come top-down from your managing director. Regardless of the relationship he has with his HR manager, the fact stands that no one want to get sued on the basis of bullying, possible discrimination, and obvious COIs put the employer at a disadvantage IMO. I'm not a lawyer but with the right kind of documentation and appropriate escalations, if your company simply does nothing they could be liable for some level of penalty.
Now if I establish your company is to some level unprofessional, I'm not going to go and say you're unprofessional, so what you want to do first is go directly to your manager, the HR manager, and make a formal complaint. You have documentation, he himself is a witness, but perhaps he doesn't know how you feel. Make sure he knows that regardless of relative status or not, it isn't okay for them to treat you that way. He should resolve the issue himself.
If he doesn't and the bullying continues after some time: you have documented that you're being harassed (I'm phrasing it this way because do it), you made a formal complaint to HR, you gave him reasonable time to resolve the claim, and nothing was done (or his relatives just ignored him). Make a second complaint, direct it to your manager, and CC the managing director. Include your documentation, and the fact that the harassment has not ceased, and that there must be a resolution. This will involve the managing director in the process, and he'll know that his involvement is contingent on a continuing issue.
Don't threaten legal action until you've consulted a lawyer and been assured you have a case. However, if the abuse is physical and you've been assaulted, as noted in the comments you absolutely need to report the assault to the authorities (police).