i have a similar issue, i have one child (DD6) and due to my age and lack of partner, zero chance of her getting a sibling. She has one cousin, who lives interstate (she last saw her for a day, two years ago). No grandparents. She cried a few days ago, "everyone else at school has grandparents etc". My heart breaks for her. i feel guilty for bringing her into the world with so little family, really just her dad and i.

She's very lonely and i have trouble organising socialising for her. She would make a great big sister, is so interested in babies.

When she goes to her dad's, she is also an only, as he has not re-partnered. i never planned for her to be an only, my plan was to have two children.

Sometimes, when she brings the topic up, i now tell her, that it makes me sad too, as i always wanted to give her a brother or sister. She is coping with that better (that it makes me sad too), than when i didn't admit to it bugging me too. Might be too soon for a 4yo to "get that" - unsure.