As we all know, Ms. Miller was a recurring character from season 4-8 in the 1983 series. I think she is a cameo and portrayed as a neighbor in the 2015 series in honor of Dody Goodman, who died in 2008.
In the 1983 series, the girls lived in the woods for the first 3 seasons and sometimes visited the boys, but no one noticed or said anything until season 4. Then it was either foster care and the girls separated or Ms. Miller, and of course Ms. Miller because they want to be together, and their tree house became a hangout area. They became closer with Ms. Miller more than with Dave.
In the 2015 series, the girls lived with Dave and the boys before the tree house. During the time, Jeanette peacefully protested against a man who wanted the tree removed. At the time, Ms. Miller didn't move to the Liberty Lane area until sometime after the protest. As we see in the episodes Ms. Miller is portrayed as a neighbor and the girls are closer with Dave than they are with her. They also come to Dave when there's a problem. If Ms. Miller really was their mom, they would be living in HER house and spend more time with her like the 80's series. I also think the girls are a little older (Brittany and Jeanette in particular in my opinion because of the makeup and MOST not all people, don't start wearing makeup until middle school/high school. I'm not talking about maturity necessarily, more like the laws but still) than the boys which is probably why they live in the backyard. However there are ziplines where the kids can go into each other's homes whenever they please, so it's not exactly an issue. I think Dave is still their guardian despite the situation in the series.