I (21F) am planning on having a distance hangout in my garden on Saturday. Two people (21M and 23F) that I deliberately did not invite are planning on showing up, what do I do?

I am living in the UK and was planning a distanced hangout this Saturday in my garden. I have quite a large garden and furniture etc for this to be possible. In keeping with lockdown rules, not many guests are able to come and so I invited my friend (25F) and her mother (55F) who are close family friends of mine (21F) and my mother (55F) too. We?ve known each other about ten years.
Here is the problem, I deliberately did not invite my friends brother (21M) and his girlfriend (23F). He has been dating her for around three years but I?ve only met her 3-4 times and wouldn?t say we were friends. There are a lot of additional reasons, but to summarise; they fight like wild animals every time we have hung out, last time I saw him alone he told me my skin is ?the colour of fresh horse ****? (I?m mixed race) and they are generally entitled and rude people, but seem to be blissfully lacking in self awareness of that. I have not yet told anyone about the racist comment but was planning to tell my friend in person on Saturday.
They seem to have found out about the BBQ as my friend was speaking about seeing me this Saturday and my friends brothers girlfriend invited herself nonchalantly which both my friend and I are upset about. Following that, the friends brother also said he was busy during the day but could ?stop by later? which was almost hilarious that he was trying to reassure that he could attend an event that he wasn?t invited to. The problem is none of these conversations have happened in front of me, only my friend, so I haven?t had the chance to assert that I don?t want them there and I need to be tactful about it if possible. They are also all currently living together so it will be noticeable if my friend and her mum leave to come hang out with me. I was going to message them directly and ask them not to come but I don?t want to upset the family friend relationship. Thing is, they are the type to just show up without even having spoken to me beforehand so I?m at a bit of a loss of how to convey this.
I need to know how to go about telling them they?re not invited, when they haven?t even discussed with me their plans to come yet?
Thanks for reading.
TL;DR - Got word that two people I did not invite plan on attending my hangout on Saturday. Need advice on how to let them know they can?t come.


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