Is it safe to wear someone else's contacts?

It is not harmful BUT
you SHOULD NOT DO IT.

Based on what you've said, you did not get fitted for contact lenses
WHY?
contacts come in different design, material and sizes, most brands have 2 sizes which they called Base Curves.
you should get your eyes fitted for contact lenses by an Optometrist, they can determine what size and what brand would be a better fit for your eyes.

Same thing for eye glasses.
try not to wear someone else's eye glasses.
When you get your prescription glasses, remember the sales or the optician use something that looks like a binocular and asked you to look straight into the light inside? they measured your pupillary distance (PD) so they can customized the lenses to put the optical center right in front of your pupil.
you might be able to see thru your bro's glasses ok. but the slight difference of prescription and PD could be "harmful" to your eyes too.


back to contacts.
if they were already opened and used by your brother
even he soaked them in the solution.
DO NOT WEAR THEM

touch lenses are layout someones eye shape, so that they might no longer in good shape the form of your eyes and would scratch your cornea. all of us desires their very own prescription to positioned on touch lenses wisely, so that they in good shape exact. some colored touch lenses enable much less oxygen to realize the attention floor, that would reason eye harm if sufficient oxygen would not attain the attention floor. continuously use a watch physician to get touch lenses, to be secure.

Nope, I donot think

If they're the same, it isn't harmful, if they're almost the same, it could be harmful, I think you should just get some more contacts, maybe disposable (I've never had contacts)? But i wouldn't recommend it, get some new or temporary ones. Hope this helps!

I don't care what anyone says, if you open a new pack it's not going to harm you. It's perfectly safe to see if you can see well in them, nothing will happen to you. However, it would be best to get your own prescription from the eye doctor.

It could be harmful - I wouldn't risk it! Even if you have the same eyeglass prescription, you could have very different shaped eyes. Because contacts rest directly on your eyes, they have a very different prescription than eyeglasses (which are about 12mm away from your eyes.) You should see an eye doctor and get a contact lens prescription.