Astra's Links
Alecto is Astra's twin brother. She is just as dependent on him as he is on her. She would back up anything he said (or secretly felt) about their relationship. Just as she knows he would back up her.
She is the type to take bloody revenge for any slight that Alecto could not brush off. She'll never doubt Alecto's ability to do anything he sets his heart or mind on. They are very on the same page to how they feel for each other.
She is the slightly more analytical, and scheming of the two, but doesn't think she is "taking care" of her sibling. They are partners in crime, and a sincerely united front.
Astra is Alecto's twin sister, she has been the only one he could rely on, ever. Despite all attempts to pit them against each other, they only clung tighter.
Alecto has Astra's back in all things. Whatever she decides, he will support her. He knows she'll do the same for him. His trust in her is absolute. He is less likely to seek bloody revenge on her behalf then she is to seek on his. He knows she'll take her own revenge and expect him to take part if she wants. He's happy to spectate or assist, whatever she wants. That is not to say that if someone has been harming her he will not take the perfect opportunity if he sees it just lying around.
Alecto has not particularly put their relationship into any specific terms. But if someone threatened him with silver... wait that's already happened. He probably lied in that case to give them advantage later. But for the purposes of knowing his character, she is his twin soul and it is like they are one person in two bodies. Any harm that comes to her, is harm also to himself.