Who should file

Fiance hasn't filed income tax or gotten any money from bio mom in 5 years since we been together. Her excuse is we have 2 kids now. (4 & 2). Also pregnant with our third. He fully takes care of his daughter when she's with bio mom in Georgia. She gets money every 2 weeks and if she asks for clothes or anything in between it also gets sent. When she's with us we don't ask for anything at all. She's been with us for 2 school years we take care of afterschool and daycare costs and everything in between. She probably has sent less then $300 in 2 years. He's even asked me to pay daycare but won't ask her for a dime. I'm telling him to file this year but he says he doesn't want her to file after and both of them have their wages garnished. Everytime I bring up the subject he gets upset with me. I've told him if that's the case if we didn't work out if it was okay if I did the same and he says no because we have 3 kids together.. Am I wrong for asking ?