Adopted with no adoption papers

I have been under the care of my mother's identical twin sister since birth since she had an only daughter while my biological parents already had seven (excluding me). However no adoption papers were made so I still carry my original father's surname although I acknowledge them only as my aunt and uncle while I recognize my foster parents as my "real" parents. This has caused me problems especially when applying for visa to visit my "foster" father (my "foster" mother has recently passed away) in Taiwan where he has migrated to live with my "foster" sister. Is there any way I can prove our relationship even without adoption papers?

can anyone answer this