I believe it should have been "I have two nieces".

Of which language(s) would that be the translation?

Correct. The sentence you translated from was the Dutch "Ik heb twee nichtjes.", so it should be nieces.

But it's a direct translation of the Dutch sentence. Problem.

I think the sentence should be left as it is, but disconnected from all sentences with different translations, like Dutch (and maybe Norwegian)? Not sure, which one's should go...

So one of the "nieces"-sentences was the one responsible for the erroneous thread. I believe one of the sentences, together with all the others attached to it, will have to be unlinked here.

Yes, just now saw Vortarulo's comment. Indeed, I believe we should simply unlink - but which sentence? I don't really know. Dorenda translated from Dutch, and the Dutch clearly says "nieces".

I don't really understand the problem. I translated from "Ik heb twee nichtjes" which can mean both "I have two cousins" and "I have two nieces". There also is a sentence "I have two nieces". If something needs to be linked differently, then please tell me what needs to be linked with what. I can see that at least part of the linked sentences are valid translations, and besides that this is a correct English sentence, so I don't see the need to change this one. :-)

I must have been mistaken:
cousin - the son of your uncle or aunt;
niece - the daughter of your brother or sister

do both have the same word in Dutch: nichte ?
After all, these are two terms distinct from one another.

And going over the Tatoeba sentences, I also found another term: kuzijn.

I didn't know either. German and Dutch seem to be different in this regard. While "Vetter" really is an older word for Cousin, "Nichte" can only refer to one's sibling's child in German.

Can't a cousin also be the daughter of your uncle or aunt? :)

In Dutch a nicht is the daughter of your uncle or aunt or brother or sister. (Apart from that it can also be an unkind way of calling a gay person, but that meaning is very unlikely in the sentence above. :-)) There is a word "kozijn" indeed, but it's (in this meaning) only used in Belgium and it's, according to the dictionary, used for males. I don't even know if it's a nephew or a cousin.

Cousin in English, to the best of my knowledge, can mean son or daughter (i.e., male or female) of your uncle or aunt.

Nephew is the son of your brother or sister, and niece is the daughter of your brother or sister.

I was sure these meanings also exist, mutatis mutandis, in the Dutch variants.