If you're talking about when Rose was young - I see no reason to disbelieve what he told Mary later - that he saw it as a trap set by Sirus, and couldn't figure a way out. If he'd said she was his sister, he probably would've been killed himself and wouldn't have saved her anyway.
If you mean just now, why didn't he save older Rose - it was implied (by both Euchre and Rose herself, earlier) that the manor's defenses prevented him from entering uninvited while the power source was still up, and would have turned him into a chicken.
Clovis was able to avoid that when he broke in, but only because he had a super-powerful magical artifact that temporarily made him more powerful than a dragon.
Raine DID say that he soon rescued them from slavery after he helped Trace, though. Now, that could also be interpreted as him "hedging his bets" (saving Mary in case he needs an ally against Trace later, while having Trace believe he was on his side), but still.Why would he willing join Trace's coop, knowing full well what would happen to Mary?
I expect the comic will answer that in due time...Why does he want Raine to remain powerless? Who's the next person to end up being manipulated by him?
DOES Sirus get on Euchre's nerves though? I haven't seen any evidence of that. In Trace's flashback on the 20th anniversary image, we saw Euchre standing next to Sirus saying "we work well together".Funny enough, the only character that seems to have Euchre completely figured out from head to toe, and the one who seems to actually get on Euchre's nerves, is Sirius, who ironically is also a character that we know practically nothing about.
Although, we HAVE seen Sirus quite clearly cause Euchre distress when he manipulated him into having to choose between his sister's life and revealing his identity.
I agree that we don't know very much about Sirus, but we still know something. We know that, up to now, in fact ever since the very beginning of the comic, he's always intervened on New Trace's side. Euchre also agreed with this when he told Rose that Sirus was an ally, "for now". We also know that on at least one occasion he's done this in a really evil and uncalled-for way (gruesomely killing the Basitin "Muscle" general, who actually seemed like a decent sort, because he was in the way of where Trace's friends "needed to be"), and of course we know that he's the one who created the situation with Rose, and argued with Mary AGAINST letting her live.
For all that he's on the "good" side for mysterious reasons of his own, he seems clearly like a way worse person than Euchre. Euchre is manipulative and self-serving, but likes to present a kind demeanor and tries to neutralize any evil effects of things he's done (even if it's just his way of "hedging his bets" as he calls it). But Sirus, when he sees no reason NOT to, likes to outright cause others pain.