I use these Philips Avent storage cups to store my milk in the fridge. I sterilize them after every use. is this ok? Do these cups have a certain "shelf life" for repeated sterilization?

Are you using these storage cups? They are meant to withstand repeated use and sterilisation. According to their FAQ page, you don't have to replace the cups but it is recommended to inspect the storage cups before each use to make sure there is no wear and tear. Replace the lids or cups at the first sign of damage or weakness.