Can grad Students date each other?

As long as you and your partner arent put into any boss-subordinate professional relationship, it is nobody elses business what you are doing. So, it is perfectly fine to be in the same research group, the same class, etc., as long as it is not a direct teacher-student relationship.

Can a relationship survive a PhD?

Relationships are incompatible with PhDs, seems to be the conclusion. Put most simply, being the partner of a PhD student largely means accepting that the Thesis is the biggest part of both of your lives, and is a far bigger player in how your relationship goes than either of you are.