Any advice

Don't bite.

Talk about neutral topics - as: gardens and colours looking nice, pets and their funny attributes, clothing for summer, holidays, children at the shore, food/cooking in summer, birthday cake.

Talk about the garden and ask if they have had any nice meals out recently. If they try to get you into a political discussion tell them you don't have the time anymore to follow the news as you are so busy doing the garden or whatever. If you talk about trivia they will soon pick up the message. You can always say if they comment on your beige conversation that you have both mellowed in your later years.

My brother a sister have differing views, they live in the US she is Democrat, he is Republican and don’t speak. Solution is easy don’t take sides and see them separately, so that’s what we do

I’d change the subject, I wouldn’t enter into any political subject and tell him I’m here to enjoy the 60th and hoping we all have a great time, I think your brother is being very immature trying to start a disagreement,

Just change the subject. Politics are individual opinion, nothing more.

I have a friend (I mistakenly typed fiend at first - maybe I should not have changed it!) who is politically slightly to the left of Atilla the Hun. When he makes remarks I change the subject.

1. Remove yourselves from the situation when subject comes up
Or
2. This is not about the bday or the visit, it is about the lack of boundaries

Tell your sister clearly politics is not a subject to be raised
If she does, trip and visit is over for you

Depending on her reaction:
1. Yes, I want to have a nice visit and I won't do it
2. Your sister fights back and complains or denies she does that

If 2, i would cancel the visit

(you're)

It's the same with a friend - so I just refuse to discuss politics with her. She always says something, so I reply 'Well, your entitled to your opinion but I really can't be bothered to argue with you' - then I change the subject.

1. Reply "Oh really!" Say nothing else regarding the remark. If more comes just Ahh or Oh I see or Mmm with a thoughtful expression!

Alternatively just look at him and say nothing. Or say Anyone want a cuppa ...

In other words just dont rise atall.