The G27 understandably has some increased felt recoil compared to her issued service weapon (G35?).
It also has a smaller grip.
These two conditions could be a factor when it comes to her grip stability and technique, both potentially being adversely affected by increased felt recoil.
Typically, when a Glock slide locks open EARLY (or prematurely), the experienced instructor familiar with Glocks, and/or a Glock armorer, will look for the following probable causes:
Improper hand position (Generally meaning one or the other of the shooter's thumbs is to close to the slide stop lever and is hitting it upward under recoil. It could be either the dominant thumb being too high and close to the lever, or the support thumb being 'crossed over' the dominant thumb and being too close to the lever ... both of which requires a shooter grip change.)
Reverse tension on slide stop lever spring (Generally the result of improper installation of the slide stop lever, which places the spring on top of the locking block - instead of below it - reducing necessary tension of the spring upon the lever.)
Damaged slide stop lever (Self explanatory, requiring replacement.)
If the last two conditions were eliminated by the instructor, then it seems possible her grip technique may require correction when shooting the harder recoiling smaller Glock .40 pistol.
This is something which would more properly be addressed by her agency's firearms training unit than someone online. Having her right there on the firing line gives them the ability to closely observe and consider both her shooting technique and the actual weapon. Offering extra range time and even remedial training is something for them to consider, within the policies and practices of their agency. She may have to ask for it, or they may require it, depending on their evaluation of what's actually occurring with her.
Remember that felt recoil is often one of the most subjective issues discussed among firearm users & owners. People very often experience, perceive and describe felt recoil differently.
I've often been asked for my opinion when it comes to some of our folks choosing between a G26 (9mm) and G27 (.40 S&W) as an off-duty weapon. When asked this question, instead of offering my opinion I generally recommend the person try both guns out on our firing line.
More often than not the person will choose the G26 after spending some time on the firing line, and the reason offered is some variation of controllability, reduced felt recoil and improved recoil management. Granted, some of the folks are not 'gun enthusiasts', and may be a bit sensitive to felt recoil, but a number of them have been men of some firearms skill and experience, who own other firearms, but who still find the lesser recoiling G26 to be more to their liking.
Now, if your sister is issued the G27 by her agency, and she's required to demonstrate competence and acceptable skills when it comes to handling, manipulating and shooting it, she should ask the firearms training people at her agency for assistance.
FWIW, if your sister is issued a G35 by her agency, this wouldn't be the first time I've heard of it being done. One time during a tactics/shooting class conducted for plainclothes and special street enforcement units I met a couple of guys who had been issued G35's by their agency. Naturally, though, the G22 would be a more commonly encountered 'full-size' model in LE circles. Not too hard to imagine that someone might perceive a bit of difference when it comes to felt recoil when going from the largest of the Glock pistols chambered in .40 S&W (and who knows what connector was ordered by the agency, too?) to the smallest, you know.
Just my thoughts.
