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I think as I have the basic package on here I can’t search other family trees on here? I’ve just viewed the record transcription of Eustace M and mothers name is Vinson no more details. Which could be Alices child as born in Plymouth the next town to Tavistock or one of her sisters children? Or someone else’s entirely. How do I find out further please?
Thank you for all of your help

You can't look at anyone's tree on GR without the tree owner's permission, no matter what level of membership you have.

And a search on "Search all member trees" (under "Search" at the top of this page) shows that nobody has Eustace on their GR tree in any case.

As already said, to find out if Eustace is Alice's child or not, you 'll have to buy a copy of the birth cert.

Register free with the GRO site - https://www.gro.gov.uk.
Then place an order for the birth cert, carefully quoting all the reference numbers, etc shown on the transcription which you've viewed.

First name(s) Sidney Edward
Last name Vinson
Death year 1918
Death date 06 Nov 1918
Occupation Porter, Traffic Department, Horrabridge
Railway company Great Western Railway
Railway department -
Service number 865602
Rank Gunner
Unit 122Nd Brigade
Regiment Royal Field Artillery
Cemetery or memorial Cross Roads Cemetery, Faontaine-au-Bois
Country Great Britain
Notes
Husband of A Vinson of 10 Taylor Square, Tavistock
Photo Yes
Information source St Paul's Cathedral Order of Service 1919
Magazine reference Great Western Railway staff magazine 1919/03/46. Photo: 1919/06/109
Archive National Railway Museum
Record set British Army, Railwaymen Died In The Great War

I posted the details of two of the sisters' marriages - didn't you notice?
Here's the third

Name: Phyllis M Vinson
Registration Date: Jul 1945
[Aug 1945]
[Sep 1945]
Registration Quarter: Jul-Aug-Sep
Registration district: Tavistock
Inferred County: Devon
Spouse: Raymond James
Volume Number: 5b
Page Number: 1022

Name: Phyllis May James
Death Age: 82
Birth Date: 23 Nov 1924
Registration Date: May 2006
Registration district: east Retford
Inferred County: Nottinghamshire
Register Number: 2A
District and Subdistrict: 686/1A
Entry Number: 210

Thank you for all of your research and help I didn’t have Phyllis’s husband and a bit more information on Sidney Vinson I didn’t know.
I will register free on gro and go from there.

When ordering the birth cert, make sure you quote all the details EXACTLY as given in the transcription.