I would TRY to let it just wash over you. She is probably purposely focusing on your aunt, but I would think that once your bub is born, either your nan will fall in love with bubs, or it's her loss. My sister has had two kids very young, and she had the same response, basically total ignoring of her being pregnant. (mainly by aunts and uncles) I find now that the bubs are here, that they don't go out of their way to acknowledge her or her kids, but when they do see them they can't help but coo over them as they're adorable.
Once your baby is here it will also become normal to them. It is so hard to imagine anyone with a baby before it's born, let alone someone so young, so maybe they just need time to adjust.
Just be happy with yourself and enjoy your pregnancy and let those who are accepting you enjoy it too. Once baby is here they will find it more normal. ( I couldn't have EVER imagined my sister with one not to mention two kids, but it's all very normal now. her bubs are 2 years and the other is 3 months).
Good luck
xoxoox