Hey OP
I haven't read through all the replies to you so someone may have already advised you
First of all you want to find out has the probate actually been submitted yet? Or is the solicitor dealing with that still putting everything together?
The agent needs to ask the solicitor dealing with the probate these questions. If it hasn't been submitted, can they offer an indication of what time scale they intend to submit it by?
If it has been submitted, how long does the solicitor expect it to take? Last I checked probate was taking 6-8 weeks to return. Note the solicitor dealing with the probate won't be the same person dealing with the conveyancing, also there is a possibility one of the siblings could be dealing with the probate themselves with the help of a solicitor, but probably not.
Ordinarily, the sellers solicitor wouldn't issue draft contract papers to your solicitor until after they have probate as technically they the siblings don't own the property until probate is obtained. However, if you wait until they have probate then all the legal work for your purchase will still need to be done, so in order to save time I would personally instruct my solicitor to begin work ASAP.
Your solicitor can't begin work until they have the daft contracts though. So you need to tell the estate agent to contact the sellers solicitor and ask them to forward draft contracts right away so your solicitor can get on.
When your solicitor has draft contracts or in the very least the title, they can request searches (on average these take three weeks but it depends what area you are in, they could be significantly faster or much slower).
If for some reason the property isn't registered as someone suggested above, this just means the title hasn't been electronically registered and the sellers solicitor will need to get the original title which more than likely is held at another solicitors. It takes a bit longer but shouldn't cause a problem
Hope this helps