At times, I think Isla is, too, but I really think she'll show more Slytherin-like characteristics as she ages (at least with being determined). There are a few things I'm thinking, though:

1. Will Isla even recognize him as being Slytherin? I can see her saying, "Some old man's lost in the castle!" She's purerblood, though, so I'm also thinking she would recognize him.

2. I think Isla would be in shock, too, but more so because there'd be a dead man walking around. One that's not a known ghost and should be in his portrait.