Bit of an ever decreasing circle potentially.
If she is planning on increasing bf to fully bf, then ideally, the baby needs to be weaned off the shields. They can prevent proper stimulation of the nipple, therefore reducing milk production.
However, if she was little at birth and finds it difficult to latch on, then the shields could help.
If the shields work and your SIL is doing okay with them, then I woudn't worry.
My DNephew was only fed using the shields as he just couldn't take the nipple properly. It was the only way sister could feed him.
Obviously, if your SIL is mix feeding, the less she breast feeds, over time, milk production will decrease as she decreases the breast feeds. If she say, bottle feeds morning and night and breast feeds in between and keeps it the same pattern , then the breasts will establish milk for the times inbetween and that could work fine.
If though, she's stopping and starting and changing timings when she either bottle feeds or breast feeds, the breasts will always be making different amounts and it will get confusing.