Tresaderm or the equivalent would have been a better choice than mineral oil for your cats. Since they all groom each other there is a possibility for each of them to ingest a bit of mineral oil and sniff it. I doubt if there was enough to cause harm for just one treatment, but please ask your vet for Tresaderm.
Incidentally, ear mites cannot be seen with the average human eye. It might be best to have your vet perform a cytology on the discharge from the ears to determine exactly what's in there. A yeast infection may have a very similar appearance to an ear mite infestation. These can be visualized and differentiated easily under the microscope when the cytology is performed.
I don't think mineral oil would be harmful, they wouldn't be able to inhale it if given in their ear canal so can't see it being a problem. HOwever I did come across a recipe in a holistic book I have and I will copy it out for you....
This ear remedy combines the anitbacterial elements of garlic with the soothing properties of olive oil(do not allow them to lick the solution, garlic isn't good for them to ingest). this can be used to clean ears as well as smother a minor infestation of ear mites for heavier infestations consult a Vet.
2 cloves fresh garlic
2 tablespoons olive oil
2 cups water
3 drops oil from a vitamin E soft get capsule
chop the fresh garlic, place in a small bowl and add enough olive oil to cover the garlic. place the bowl over some just boiled water or heat in the microwave for 30seconds.....let cool to room temp if your going to use immediately. otherwise its best to let this sit for 5-7days in the refrigerator and allow the garlic to infuse and release its medicinal consituents. before using, strain the oil to remove the garlic and add the 3 drops of vitamin E. this mix will keep up to 3 months in the fridge..
to use, saturate a cotton ball and squeeze into the ear canal every day for a week to kill the mites than 3-4x a week for at least 6 weeks to kill the larvae and prevent recurrence.
good luck.