Just been invited to SILs hen next year (September) but DH and I are planning to start TTC in January. It’d be really fun but not sure I fancy it if I’m pregnant (it will be boozy!) and will involve flying - and in any event in the very unlikely case we fall first month I could be giving birth at that point 
DH and I have agreed we won’t tell parents about TTC - they’re lovely but I don’t fancy them doing ‘is this the month’ every month and would rather give them a nice surprise than have them asking about how we’re getting on 
If it were a friend I’d quietly confide in them and ask them to keep it v under wraps, but it feels unfair to tell SIL and then tell her not to mention it to her mum. I think she’d try but it could come out by accident and it’s not her responsibility to worry about it/keep the secret.
Could say ‘can’t afford it’ but her hens are sweet and really working to keep costs down and I think she’ll be disappointed. I really don’t want her thinking I don’t want to join her! Given it’s 11 months away not sure I can claim prior arrangement either. WWYD?