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He thinks of Nobunaga and his wife Nou Hime as his (foster) parents, so foster aunt would be closer to the mark.

Game, anime, manga, live action...
The series is looseleaf historical, and Nobunaga is married to Nou Hime, who I've officially posted as not appearing, since she's tending to the Sixth Heaven. Oichi is his sister. Ranmaru sees Nobunaga and Nou Hime as his parents, though he actually works under the two and historically one of Nobunaga's retainers (and lover, though that goes unmentioned in the series due to age difference). As such, Oichi is his foster aunt.
One screwy family, am I right? And since the series doesn't even connect to itself, making it hard to figure out what is and isn't canon, the game has Nobunaga killing Oichi's husband when the former goes to shoot her. He tells her that it was her fault, and the second game has her falling apart mentally, killing Ranmaru and Nou Hime. In the anime, soldiers under the command of Mitsuhide, who also works under Nobunaga, kills Nagamasa, though their target is also Date Masamune, who is hit, but no one realizes it until he's passed out several miles away and falls off his horse (slo-mo shot). Ranmaru is told to find a new master and disappears, Nou Hime killed off by Oichi again, this being the first appearance of the Shadow Hands in the anime. Before the third game, and through a good amount of the first season of the anime, she uses a double-bladed naginata, though she never uses it in the anime.

Confusing, I know. But basically, Ranmaru is the foster child of Nobunaga and Nou Hime, which has Oichi as his aunt.

having never played that game, the best I can think of is that she was insane at that point and didn't recognize him at all, though in the third game she mistakes Tsuruhime (female archer) as Ranmaru.

A country at war, technically no one involved is innocent
In the third game, her red path takes her to join Lord Happy (Ieyasu) who asks her to wait, but she fights anyway. He appreciates the help, at least. It's probably the only true happy ending she gets in that game, unless you count when she fights Nobunaga and goes to the afterworld with him, and he just kind of...goes with her.

Ranmaru is crushing on Maylene XD

psssht, the series is yaoi-heavy, with a lot of fanservice

That would be fun, since that means that Nobunaga can't kill Maylene, because he doesn't like Ranmaru being sad, and if Ranmaru is sad and moping about, then he's useless and has to die, good as he was.
So, if we do it, then Maylene cannot be killed by Nobunaga. Oichi still might. On accident, so hope she sees what's going on between the two and merely knocks her aside. Maylene being forbidden to be killed by this means a measure of protection for Annabelle as well, since the two are siblings.

It just got complicated, which will make this even more fun. Woo!

probably not much past his inept attempts to flirt and Maylene running because she took it the wrong way, especially at first. Farthest I see it going is holding hands and Ranmaru protecting Maylene from Nobunaga and Oichi. They might end up fighting together as well.

Harumi and Suki are also in that immune phase, since Oichi has latched onto her, more or less, but if Harumi and Nobunaga end up fighting, family takes priority for her, mostly through the demons the two have.

The fact that he's younger than everyone else makes it kind of funny.
I'm curious to see how flustered Annabelle will get over him, since she's crushing so hard. Oh, and next time Maylene and Ranmaru cross paths, he'll apologize and let her know about how he sees the 'fainting like a girl' line as a joke, since the only female he's really been around is a gunner and is almost as ruthless as Nobunaga. She didn't care that he destroyed her home, that's how bad she is.