Partner may have another daughter?

just wondering if i am being unreasonable here,my partner & i have two kids,prior to our being together (over 7 years ago) he was in a relationship which ended,she slept some other people immediatly & then found out she was pregnant (n& unsure of the father...anyhow...someone has signed the birth certificate to say he is the father & acts as the father etc however there is a possibility that my partner could be the father, i have always known about this...my question is though....his family speak of her as though she is his daughter,count her as a grandchild etc,...& have told other grandchildren about her?! there is no evidence to prove 100% who the father is (there is no intention of DNA testing at this time), this puts me in an awkward situation as i dont beleive that it was anyones choice but ours to disclose this information (the nan has told the other children) especially since there is no reason to at this time, the child is not in our life...i feel undermined & disrespected?