Given how Carrie and Big did talk extensively about their finances when they were buying a place together, and given the nature of his business, I would be highly surprised if he had not updated his will to include Carrie.
Also, I imagine he has probably left Gloria something nice, but his entire estate, doubtful.
Re. Natasha being the mother of Big junior and Carrie not knowing, I can't imagine this either. However, Natasha does appear in the series, so maybe Big did leave her something, her and Carrie's paths cross unexpectedly or maybe even Natasha contacts Carrie having heard of Big's death. Who knows, we will all find out soon.
I just don't get this need to paint Big into some kind of awful person or revenge affairs between Natasha and Big. Admittedly Big was an arsehole at the outset but he has clearly mellowed with age. Given the title 'And Just Like That refers to Big dying ('and just like that, Big died'), maybe, just like that, Carrie meets someone and realises it's time for a new love. It happens. Life can change in a heartbeat.
Oh, and to those who don't understand why Carrie didn't do anything as Big was laying there dying. Having been in a similar position with my dad, I just sat with him and held his hand (although to be fair, my dad was terminally ill anyway and would have wanted to just slip away peacefully, which he did.)
Gosh, that's a lot of words 