"Tomorrow we are going to go to my sister's house."
Translation:Amanhã vamos para a casa da minha irmã.
And I'd say it is bad English to say "going to go" as well. I got it wrong when using 'vamos ir', but actually I would say that's exactly what the English text says -- I've suggested to cut the last "to go" from the English text to make the sentence ok & be the same in both languages.
Indeed, you're right, I just feel it's the same awkwardness as "vamos ir" in Portuguese. (In your example, I would probably have said "I was about to go, but the weather was (too) awful", btw.) Neither English nor Portuguese is my mother tongue, so I'm definitely not God here, I just think Duolingo is virtually asking me to write "vamos ir" in PT when they present the "going to go" in the English exercise... :-)
Yes, isolated (like saying "I'm going to leave") I agree that "I'm going to go" is standard and ok, but to say "I'm going to go to my sister's house", which is the case here, is one "go" too much and would better be "I'm going to my sister's house" or "I'll go to my sister's house" or even "I'm about to go to my sister's house".
Well, that's my 5 cents anyway :-)