I'm really sorry to hear that. Hope all your family and friends are alright.
_____

「姪ちゃん、前に結婚したおねえちゃんのベビー?」

First, unlike 母ちゃん and 父ちゃん, you don't address your own niece as 姪ちゃん
. You would address her with her actual name or nickname. So the addressee of this sentence is NOT the niece. It is someone's niece but not the speaker's. I will come back to this in a second.

Now, think about who the speaker is. When I read the 「前に結婚したおねえちゃんのベビー?」 part, I immediately knew that the speaker was not part of the family in questioon. Agreed? The speaker does not even know who this girl's mother is. S/He doesn't seem to know much about the family.

Who is the addressee? From the context, it is clearly a member of the family because the addressee is someone who could answer the question being asked: "Who is the mother of the girl?" So, the addressee is related to both 姪ちゃん and お姉ちゃん.

So, we can safely say the following so far:

Speaker = outsider to family
Addressee = family member
Subject of sentence = 姪ちゃん

Let's add words to the colloquial sentence so that Japanese-learners can see the structure.

「姪ちゃん、前に結婚したおねえちゃんのベビー?」
becomes:
「姪ちゃんって、前に結婚したおねえちゃんのベビーの 事ですか?」
って　＝　は

姪ちゃんって = As for the niece that you just mentioned

We now know that the addressee would have to be the uncle of this niece, wouldn't it?

Thank you for your concern, we are all fine.

I understand now why it is your niece. I didn't know you address differently to niece. I thought member of the family you address the same way. Also おねえちゃん tricked me. I guess the way that he used ちゃん made me immediately think about member of the speaker's family.