I chose ma'am because it's respectful (like obasan, and it fits with Sanae asking for a favor) but still indicates that Sanae has called Kochiya old. I don't think "old lady" works as well, but I admit that there's probably better choices.
Auntie seems the most accurate. Plus "old lady" is "obaasan" with the elongated "ah."
ahPlease let me, auntie.I'm begging you! C'mon, you ask too.*cracking*We're back. Hm, who's the kid?Welcome back.Stare~I think she's probably from the village, but I took her from a pervert at the base of the mountain...A pervert?If it's possible, I'd like to let her spend the night here. If she is from the village, her family will be worrying. Wouldn't it be best to send her home?For a human to have this magic is definitely...