How young were you thinking for her? You have him in his forties, right? So have her be 5-6 years younger, or maybe a little younger still which adds to the protective brother feelings of why they're close too. Because with a single mom that's a little neglectful, especially with the, keeping a roof over their head mentality, she'd be a small handful but also happy to be kept safe under his wing.
And also she'd definitely tag along with him for a free vacation in sydney. Get out of LA for a little and since he's single, she says someone has to live with him to make sure he's actually taking care of himself (coupled with LA isn't exactly a cheap city to live in)