Do I have to locate a lost relative who is named in the will before it can be legally probated?

If you offer the Will for Probate in Solemn Form (which is the best one to use), then you will have to at least try to notify your sister. However, if she is really not findable, then you may be able to try notice by publication. The Court may also require you to hire an heir finder service. If you offer the Will for Probate in Common form, then you don't have to provide her with notice, but she will have 4 years to come back and try to challenge the Will before the probate is binding on her. That's why Probate in Solemn Form is usually better. As for her bequest, if you get the Will admitted to probate and she still can't be found, the Executor may have to turn the bequest for her over to the probate court. But it is possible to move forward. I strongly suggest that you hire an experience probate attorney to help you, because it won't necessarily be easy, and the court isn't allowed to help you with legal advice.