Unfortunately for Sirius, Andromeda has the disadvantage of a number of years away from her cousin. Clearly they have diminished her ability to understand exactly what he's doing.

Once Sirius apparates, she lets out a breath and then sighs.

"Finite incantatem," she mutters, wand pointed at Snape. She looks over to Lily, glad to see that -- well, they didn't have to forcibly remove anyone.

"Now, would you like to make some attempt at explaining what exactly we walked in on?" Because, unlike some of her other relatives, she does give people the chance to explain themselves to an extent.