1) DON’T mention her size. Although her perverse streak meant that my sister positively enjoyed the stage where strangers feel uncomfortable wondering if you are pregnant or just fat, one teenage boy will always regret the moment he leaned out of his mate’s car and shouted “I hope that's diet!” to my sister as she bought a Coke. (She literally reduced him to tears. Rookie mistake, kid.)
Similarly, I have learned to my cost that even blood relations are not allowed to study the “bump” through narrowed eyes and wonder aloud if she's sure she isn't having twins.
Our father takes the biscuit though: “I’m surprised you've got so many stretch marks, because you had so much skin to begin with.” Tactful, dad.
2) DON’T even attempt to assemble an infant car seat unless you have a degree in advanced mechanical engineering, or were once a technical advisor on Scrapheap Challenge. The first astronauts went to the Moon with less protection. Glibly agreeing to have a go cost me most of an afternoon and two fingernails. Personally I don't see why you can't just wrap them in Bubble Wrap.
3) DO obey the hospital’s rules. I was unceremoniously ejected from the delivery suite at midnight when a senior midwife realised my sister had three birthing partners. (Previous staff had clearly been fooled by my skilful impersonation of a foetal heartrate monitor.) Poor Sis was forced to soldier on without my patented Rousing Medley of Show Tunes.
4) DON’T expect to win a conversation that starts “I’m so tired...”. They have the monopoly on fatigue for the next 18 years. Especially if you let slip that you are tired because you were down the pub until midnight.
