Can I use my account to do my sister taxes, right know she is in a situation where she is not able to do her taxes before the dead line april15. given me her permission

You can prepare her tax returns with her permission, but she would need to be the one to electronically 'sign' it to actually submit it.  If that is not possible, you should file an extension for her.  If she won't be able to 'sign' it by October 15th, you should look into getting a "Power of Attorney" that would allow you to do that.

I recommend setting up a separate account for her.  Although it is possible to do multiple tax returns in one 'account', it is easy to mess that up and overwrite your own information.

Online is only good for 1 return.  But you can use the same email address for 5 accounts.   How to start another return in the Online version https://ttlc.intuit.com/replies/3288574 

Thanks for correcting me, I knew you could do five returns, but I didn't remember it was per email address, rather than per 'account'.