Translation:Grandma came to our house this weekend.
You should only use 这个周末 to refer to an upcoming weekend, or the current weekend while it is still happening, OR in reference to an obvious weekend in the context, like if you are talk about a long holiday weekend.
上个周末 is when you are dealing with past tense.
来了 can also mean 'has come' or 'is coming'.
You can say 老师来了when you see the teacher on their way (to the classroom or something). You can also say 老师来了 when a teacher has come (to the classroom). To be more clear when there is no context, you could instead use 到了, for 'has arrived', especially if they are still there.
了 is a relatively difficult particle to teach, but just remember that is can usually be described as a 'change in status'. For example, the teacher was not coming, and now they are, OR the teacher had not come, but now they came.
“得看上下文” = It depends on the context!
I hope that helps!
I feel it would be more natural to take this 来了 as 'is coming'. The 'change in status' could be described naturally in Japanese as well. I just think of 「お祖母ちゃんが週末、私んちに来るんだ。」 for this Chinese sentence. Anyway, the lack of the context is one of the biggest drawbacks in Duolingo courses.
'This weekend' can also refer to a weekend that is currently happening, meaning if it is Sunday and grandma arrived on Saturday, it's still technically past. Though, grammatically this sentence is quite unfortunate, because 'this weekend' and past simple doesn't really work together, they should use present perfect.
It is not uncommon on a Monday to say to someone:- what did you do this weekend (meaning the one just gone). Reply:- Grandma came to our house this weekend. But by Wednesday this usually refers to the one coming up, but of course the context is changed because you would ask What are you doing this weekend?
I believe that 家 is a much broader term, therefore a sentence "Grandma visited us this weekend" should be accepted, because that's the definitive meaning. Because of these ambiguities, Chinese Duolingo lesson is not covering many of the possible translations yet and because of that, I can't pass the checkpoints and that makes me sad :(