Even if we pretended that Harry wasn't close with his brother and sister-in-law (which, by all reports, they are close), it would be absolutely absurd to think that Kate's due date wouldn't be taken into consideration when planning a wedding. This is a family that is meticulous in planning, and deliberately ignoring Kate's due date would be absurd. It would send the wrong message to the press and public, and feed into the "Kate and Meghan hate each other" story that the press is always itching to write (because nothing beats playing two women off each other).
If Harry and Meghan have an early 2018 wedding they're going to avoid several dates:
- Kate's birthday (9 Jan)
- the Queen's ascension day (6 Feb)
- Kate's due date (sometime March-April)
- Easter (1 April)
- Charles and Camilla's anniversary (9 April)
- the Queen's birthday (21 April)
- William and Kate's anniversary (29 April)
- Charlotte's birthday (2 May)
- the Trooping (9 June)
- the DoE's birthday (10 June)
I would also expect them to avoid Andrew, Edward, Sophie, Zara, and Eugenie's birthdays, and Peter/Autumn and Edward/Sophie's anniversaries, which all happen in that timeline.