Surname Change Minor

Hi po and Good day, pasensya na po did not have enough time to go through the forums to see and check if may nakapagtanong na about this, I also feel like mas importante kung mas kumpleto yung detalye. Anyhow, on to my concern. Yung pinsan ko po was in a domestic partnership for 7 years. Then this year naghiwalay po sila nung babaeng kinakasama niya, dinala po nung babae yung 1 yr and 6 month old na anak, and ngayon po klineclaim niya na hindi anak ng kuya ko yun and wants to change the child's surname sa apelyido ng current partner niya. Nakaapelyido po kasi sa kuya ko ang bata and hindi din po sigurado ang paternity ng bata and we feel na nagsisinungaling yung babae by claiming na yung bago niyang kinakasama ngayon ang ama ng bata at hindi po ang kuya ko(ayaw po niyang ipaDNA test yung bata). Ano po kaya ang legal options namin? Eventually gusto po sanang makuha ng kuya ko ang custody pero right now gusto po namin na hindi mapalitan ang apelyido since we cannot just take the girl's word regarding the child's paternity. And finally mapapalitan po ba nila last name without involving my kuya in the entire process? Thanks po and God bless.