Firstly, try and stop worrying. You have all the tools to hand to be on top of the situation :)
Speak to your DP. He needs to realise that 'not rocking the boat' is not an option. The boat IS being rocked - by his brother! Have a serious chat about how upset you are - you are dreading your baby coming - that's not right! Tell him that you will be speaking to your midwife and HV about this in order to put your mind at rest should his brother do anything to harass you once the baby is here. You WON'T be starting any process or complaint - you'll simply be making professionals aware that there might be a problem.
Speak to your HV and MW. Just talk them through the issues and say that one of the things you are afraid of is that he will make malicious complaints about you, perhaps to SS. They will have heard it all before, don't worry. Get them to put a note on your file, and do the same with the GP. Then, if he DOES make a complaint, your notification of his behaviour will be there FIRST. It will look sensible and responsible of you to make them aware of this issue.
Don't get het up over the idea of SS. Yes, you shouldn't have to deal with this at all, but his brother exists and is making this trouble - so you just have to be rational and deal with it as it comes up. Far better to take the bull by the horns NOW and make the professionals aware of this individual. There's absolutely nothing for you to worry about here - except the worry itself if you see what I mean. Deal with it now by making GP/MW/HV aware and you will feel much more in control. They might also be able to give you some good advice on what to do if he DOES call SS. They're there to help - go to them.
Finally, good on you for thinking injunction if he does threaten you. I'd go further and contact my local police unit and ask their advice now. Again, you don't have to do anything, but knowledge is power - and I'll bet your DP's brother would soon turn tail if next time you had contact with him, you coolly informed him that SS and the police were ALREADY aware... of HIM!
Best of luck having your baby :)