Bernwyna can be a modern form. Source: https://oswinkinsey.wordpress.com/ [See document titled "English Compound Names". 'Bernwyna' is at page 114])Based on endings of other Old English feminine names ending in -wynn (like -wyn, -wynne, -wen, -wena and -wenna), I imagine that Bernwyn, Bernwynne, Bernwen, Bernwena and Bernwenna could work as other forms. Unfortunately I can't find sources showing these though.Bernwin may be Latinised to "Bernwina" and "Bernwinna".
Based on endings of other Old English feminine names ending in -wynn (like -wyn, -wynne, -wen, -wena and -wenna), I imagine that Bernwyn, Bernwynne, Bernwen, Bernwena and Bernwenna could work as other forms. Unfortunately I can't find sources showing these though.
Bernwin may be Latinised to "Bernwina" and "Bernwinna".