Familysearch.org (FS) has the following information available which I hope may be of use
John de Mowbray 2nd Baron Mowbray FS ID LYWF-C7S born 4 September 1286 Epworth, Lincolnshire, England – christened 11 September 1286 Chamberlain, Norfolk, England –died 23 March 1321 Yorkshire (hanged)
Spouse Alina de Braose FS ID LVX3-2TM born June 1290 Bramber, Sussex, England – died before 20 July 1331 Stringston, Somerset, England
John & Alina were married before 1305 in Swansea, Glamorgan, Wales this union produced 9 children 3 boys & 6 girls
Source https://www.familysearch.org/ark:/61903/1:1:QVGH-14P5?from=lynx1UIV8&treeref=LYWF-C7S
John’s Parents
FATHER: Roger de Mowbray 1st Baron Mowbray FS ID LH6L-FPM born 1257 Axholm Island, Lincolnshire, England – died 21 November 1297 Ghent, East Flanders, Belgium – buried November 1297 Fountains Abbey, Harrogate, Yorkshire, England
MOTHER: Rohese de Clare FS ID LH1Z-ZZ4 born 17 October 1252 Tonbridge, Kent, England – died 1316 Hovingham, Yorkshire, England – buried 1316 Fountains Abbey, Harrogate, Yorkshire, England
Roger & Rohese were married 1270 in Thirsk, North Riding Yorkshire, England. They were the parents of 11 children John was the 6th eldest
I suggest you go to Familysearch.org to check ALL the information that is available
NOTE: I am passing this on for your information – I can only be guided by what is on that site, correct or not. Sources may/may not be available
Cheers Ross Geissmann