My student who you had a private with is doing well in class though she still does not do forward bends for the time being. You asked her to ask me to evaluate her s/i or s/l? Well, I thought you meant s/i and it was fair, in other words the bumps did not move up or down. Question: what does her s/i mobility have to do with her unstable L4/5?
Normal for hip flexion is that the sacroiliac joint moves upward as the knee goes above hip level. With both immobile as you are reporting there is either a frozen joint or it may prove to be unstable with internal and external hip rotation motions. To find out you can put her in one and retest and then the other and retest for evenness of static position as well as dynamic with hip flexion test. Lack of mobility there will require spine to have extra mobility, and more instability.