Some folks aren't social, some have health issues etc. Maybe with the last child, she's dealing with post-partum depression and things are pretty sucky and she just doesn't want to deal with people. (I had PPP (post parturm psychosis), and am still on meds, even at one year I tried to wean off, and it was bad). Maybe there are marital issues and the husband doesn't want to talk about it. There's a lot of maybes, and personally I'd just take it as it goes. If a neighborly relationship is important, build it with the father and kids, and if the wife never comes out, then it is her loss, but it might be something she doesn't want/need right now.
As one who can get somewhat anti-social, while I would appreciate the efforts and eggs etc, I'm very sucky about reciprocating, and even feel worse when I don't know how to repay kindness from others. I'm sure some of our neighbors think I'm a recluse sometimes, but I'm ok with that. I'm just happy that soon we'll have eggs to give away to those who are kind to us.
Some folks need social interaction, some don't. I'm one that doesn't - I'm perfectly happy with my DH and DD, and online. I get enough interaction with humans at work
I would keep the door open to her for interaction, but I'd suggest just letting it go and accepting that maybe she just isn't social. Keep a nice relationship with the dad and kids, and if and when she appears, include her in.