Would your sister be starting in a Bachelor program or with a Masters/Ph.d? If she has studied something before that is not psychology (for example philosophy or economics), then they may not allow her to change into a Masters/Ph.d program. If she is just starting then her chances are good to get in.

Another thing about the market. By the time she gets out of school with her degree demands will have changed. If she likes psychology and wanted to be a shrink then she should just go for it.