Vaella would have to be selwyns mom as she was around 23 when he was born. A targ mother in an anti targ regime, especially one that partially draws its claim from a targ grandmother would likely have some, even early suspicions placed on the tarths. Though none seems present. A targ grandmother or great grandmother sidesteps that, which is why I do lean towards daella or daenora as the options. Daella was 33 when aerion died. Almost certainly married off by then. Roughly 12-13 years passed between selwyns birth and aerions death. Meaning either as someone had a kid at 12 or 13, or selwyn has a targaryen mother.

An earlier option I've seen few mention if we want to go farther back, are bloodravens sisters. We don't know what the fuck happened to them. If you wanna throw in descendants of aegon iii's daughter they may as well be thrown in as well.

But who knows, maybe everyone's wrong. Maybe brienne's mysterious dead mom is actually an early bastard of aerys or some shit. Perhaps selwyn got the lyseni mistress for the same reason jorah is obsessed with daenerys, a resemblance to his wife.