Sometimes she thinks that their next mistake (Lily keeps saying their, but really the mistakes are all hers) was in deciding to have no other children.
Of course, Teddy had breeched the subject once when he had returned from one of his longest and most gruelling missions. They lay together in their four-poster bed, his muscled chest pressed against her back and his large hands wrapped around her own and he asked her in the darkness if another baby would fill the loneliness she and Cepheus must have felt when he was away. Lily said no with resounding finality and true to his character, Teddy didn't push for a reason. She always loved that about him, his absolute faith in her decisions. She knows that if he had of asked her 'why' she couldn't have given him any proper reason without coming across as selfish and vain.
But if she wants to be really honest with herself, Lily knows she said no more out of selfishness than anything else. She knew she could not go through another nine months of back aches and horrible bouts of nausea that could last for days on end and she knew that another baby would most probably ruin the figure she had worked so hard to regain after Cepheus.
Lily also knew that she was not quite suited to parenting. She loved Cepheus with all of her heart and she couldn't imagine never having him but the little boy was draining. She often found herself wailing along with him when he was frustrated or naughty and unwilling to co-operate or listen to her. Both Lily's mother and grandmother reassured her that he would grow out of it and they were right of course, but Lily could not bring herself to go through that with another baby; she thinks she just may have collapsed and given up, perhaps even joined Scorpius in St. Mungo's.
What's more, whenever Lily or Teddy so much as mentioned the possibility of a little brother or sister to Cepheus, he would scream and howl and clutch at their legs, begging them not to replace him. His passionate little outbursts amused Teddy and he would laughingly reassure the little boy that a new baby was not a replacement. Lily on the other hand, was always taken aback by how vehemently the boy believed he was not cherished. She took it as an insult to herself; was she too cold? Did she not show how much she loved him? Did she show him too often how much he could irritate her?
Lily sometimes lets herself imagine what it would have been like if she had said yes to Teddy and had another baby. Cepheus would have come around to the idea eventually she thinks. A playmate and companion would have done wonders for his aversion of other children (and as he grew older, his aversion of people in general) and she thinks, judging by the once close relationship he shared with Al's little Lotte, he would have made a wonderful big brother. She indulges herself every once in a while and pictures a little girl with an ever-changing nose following Cepheus adoringly, or a little boy with hair as red as her own tugging on his hand and leading him on this adventure or that.
The 'what ifs' and 'should I haves?' become too much though. It is only when she is most confused by the crimes written against Cepheus's name in Teddy's Auror reports that she tries once again to justify her own innocence to herself. She allows herself to believe that it was quite possibly Cepheus's protectiveness of (not so little anymore) Lotte that started him on his dark path and not her own poor mothering skills or his lack of childhood companionship.
And so Lily always comes to the same conclusion; a sibling would have only driven him further way. She deludes herself into thinking that it quite possibly would have sped up the process of his isolation and descent. But if this were completely true, she wonders why images of wild curly haired little girls and boys with eyes just as mismatched as Cepheus's haunt her so at night.