Lots of things spring to mind here OP. First of all, what does your brother think, has he met her?
My instinct tells me that this woman is being distant with you in the hope of stopping you visiting when you know she’ll be there, as you’ll feel uncomfortable in her presence. The same with the flirting with your daughter, ie, make your DD feel uncomfortable, and she won’t go visit while she’s there.
I think in your shoes, rather than calling her out in front of your parents or anything like that, I would be visiting often when you know that she’s there, and in front of your parents, saying things like, oh it’s so nice getting to know you, why don’t you come over to mine one night, we can have a girly night, have a few glasses of wine, etc? She’ll probably make an excuse if things are not as she would have you believe, but if you can put her in the position where she has to turn you down in front of your parents, like, ‘Oh, I’m not much of a drinker’, then you could say, well ‘How about a movie together at the weekend?’ Come up with as many, varied ideas that you can, so that if she turns them all down, it will look like she’s being churlish in front of your parents, and they might start to wonder why she’s turning down the offer of friendship from someone nearer her own age. If that doesn’t work, and you get really worried, would your DD be prepared to make up some excuse to move in with them for a while. If they feel that this woman is filling a gap in their lives, then the chance to have their GD move in, may well see their interest in her fading rapidly.
Another thought would be to ask your DM on the quiet, if she’s your Dad’s love child, or even ask if she thinks he’s having an affair with her, pointing out that while there is a big age difference, it’s not unheard of, and it worries you the way she’s always hanging around him, etc., maybe give your Mum pause to wonder about this, then if she thinks she might be a threat, she could well become unwelcome really quickly.
On the other hand, if your DB feels the same way about her, you could always get him to warn her off, being really nasty and threatening, but obviously not where your parents can see or hear. Then if she were to run to your parents and tell them that he’d threatened her, you could ask when it happened, and say ‘well that’s a lie, as he was with me on that day/time’
Hopefully some ideas there for you to think about.