If she's 13 there is a good chance she's already having periods.
If she's hiding it from you, that might mean she isn't comfortable talking to you about it.
That doesn't reflect at all on you, or your parenting. Teenagers can just be very private about things like this. I didn't tell my own mother and I can't really tell you why (I started very early, so it took her a while to cop on, and she just assumed I'd tell her because she thought she was "cool" about it).
I think the Aunt's offer is a lovely one, and worth considering. She would be a worthy stand in for your DD's mother in the circumstances and might be able to offer her some good advice, or make it feel more normal for her because it's something she has taken for granted for years.
But even if she does the first approach, that doesn't mean you should avoid the subject. Like Heated says, there are practical considerations. If you go the Auntie route I would maybe see if she can divine how your daughter would like you to deal with this. She might be prickly about it and embarrassed. It won't last forever, honestly.
Best of luck with it and so sorry for your loss.