I thought she was older, especially by this point. Though--and this is just subjective--but I feel the opposite about her father figure-I think what she needs is a mother. She's got a domineering father figure and while Bruce is certainly a much better one I think he's also comfortable for her because of his own obsessions and that's not necessarily a good thing. So I can't help feeling like no, that's exactly what Cass *doesn't* need is to make Bruce her father as what Cain should have been. Like getting things she didn't get from Cain with Bruce is great but stick with Babs for a parent.
But regardless, it's the fact that they have ignored the adoption that even at the time seemed mostly about just her and Bruce that makes it seem like an empty gesture. As you point out, we don't know what she thinks of recent events. I'm not sure if she's met Damian or if she'd care about him. Meanwhile the other kids are locked in sibling drama and everyone is treating them as such. It doesn't seem like they played Cass's reaction, such as it was, very well as a daughter losing her father story. Writers "forget" that Cass is a sister in ways they never forget even Jason is a brother. I think because they're not forgetting, they have just never thought it was true.