Interesting little statistic:1920 was the biggest birth year on record in the UK. Proportionately speaking, not even the post-WWII Boom beat it. The birth rate fell sharply in the decade that followed, but in 1920 the Bransons were in good company.
I could see Sybil wanting to stay involved/useful outside the home even if it doesn't mean holding a job. Or she might be one of those writers' wives who help with typing and editing. But I think you're right:she won't be opposed to being a homemaker.