"My sister was thinking too much about that."
Translation:Ma sœur réfléchissait trop à cela.
So, penser vs reflechir rears its head.
However, are we really being given enough context to suggest that only reflechir works with this sentence? I don't think so, but I'm happy to be corrected.
Can 'y' be used here instead of 'à cela'? 'Ma sœur y réfléchissait trop'?
While I have no idea either, it is clear that penser should also be accepted here. I assume that pensait will be added as a correct answer once enough people report it, but I've been wrong before!