Everyone prepares for a second baby's sleep, feeds, and admin. Fewer people prepare for the toddler who suddenly can't stand the sight of the baby - or, just as often, the toddler who seems fine for weeks and then falls apart out of nowhere.
Both reactions are normal, and there's a fair bit that genuinely helps.
Why it happens
A new baby is, from a toddler's perspective, a permanent stranger who has moved in and is getting most of your attention. Research on sibling adjustment consistently finds that challenging behaviour and emotional regulation both get harder in the weeks after a new baby arrives, often peaking around the two-month mark - not the first week, which catches a lot of parents off guard.
- Regression - toileting accidents, wanting a bottle or dummy again, baby talk
- Increased clinginess, or the opposite: sudden defiance and testing limits
- "Sending the baby back" comments - upsetting to hear, very normal to say
- A delayed reaction - some toddlers seem completely fine until weeks in
Before the baby arrives
- Involve them in the practical stuff - choosing an outfit, helping set up the room - so it feels like their event too
- Talk about specific changes honestly ("the baby will need a lot of feeding at first") rather than only the exciting parts
- If a room or routine is changing because of the baby, make that shift a few weeks early so it isn't tied directly to the baby's arrival in their mind
In the early weeks
- Give them a real job - passing nappies, choosing a toy for the baby, being your "helper" - rather than just asking them to be patient
- Protect small pockets of one-on-one time, even 10 undistracted minutes, especially right after a feed when you have a free hand
- "It's hard sharing Mum, isn't it" lands better than "be nice to your sister" - name the feeling without judgement
- Lower the bar everywhere else for a while - this is not the season for a big push on manners or the last stubborn nap
When to expect it to ease
For most families, the sharpest edge softens within a couple of months, though it's rarely linear - a good fortnight followed by a rough week is normal, not a step backward. Genuine security in the parent relationship, not the toddler being naturally easygoing, is what predicts an easier adjustment - so the reassurance and one-on-one time matter more than getting the logistics perfect.
It's not a competition they're going to lose
A toddler who's struggling with a new sibling isn't broken and isn't a bad omen for their future relationship - it's one of the most universal reactions in parenting, and one of the ones that improves fastest with a bit of deliberate attention now.
The two of them being closest of friends in a year or two doesn't erase how hard this particular stretch can be. Both things are true at once.