Well, in GFII after their mother died, Connie DID say to Michael that she wanted to stay close to home and take care of him.

One can probably assume that over all the years of doing just that and probably having no one else in her life (apparently not even her sons as they had no part in GFIII) she had grown closer and more protective of Michael - and more the classic Italian matriarch - with every passing day. She would have watched him build the Corleone empire to heights even their father might not have dreamed of What appears to us as a 'transformation' was more like an evolution that took place over nearly 2 decades. The only thing that's a bit hard to swallow is how she ended up with so much 'clout' on the business end...but even that makes a little sense if you consider the deep trust & bonding that may have formed between brother & sister who were raised in the same world.

In fact, Connie's 'transformation' is far easier to watch than Michael's who for most of GFIII is a clown compared to his earlier self.

Apple