it is okay to date anyone who is not a "blood relative". Anyone who is a "Step..." anything is not a blood relative. For instance, it is okay to date your: Step Cousin, Step Second Cousin, Step Third Cousin, etc. it is even okay to date a step Brother or sister, however it is slightly looked down upon by most societies.
They can date as long as they are not related by blood.
yeah.. that's creepy
NO
Your sister's aunt (who is also your aunt) is the baby's great aunt. The baby is a great niece or great nephew of your sister's aunt.
step brother
As long your dad is ok with it, its fine to go out on date with your step mom. But its very immoral to go out with your step mom even if she is young.
grand aunt
If she looks good then go for it. Aunt's are the best
He did date my aunt and kinda still do
She has cancer
Your grandfather's step-son would be your uncle, or step-uncle, depending on how technical you are getting. Your uncle's ex-wife would be your aunt, ex-aunt, or ex-step aunt, again depending on how technical you are getting. Blood-wise, she is no relationship, so it comes down to if you still want to consider her an aunt or not. If you still want to have that relationship with her, it is okay. If you don't, there is really no commitment.