Aren't you supposed to work for 3 months or something after maternity leave finishes or become liable for paying your maternity allowance back or something?
Sorry, am out of date on this but know of a couple of people who (couple of years ago now) had to find childcare for that 3 months as they couldn't afford to pay back their allowance IYSWIM.
Definitely think it is worth talking to HR in the morning, no need to drop your brother in it, talk to them and assume that you are still employed. Find out about return dates and how long you need to work so you don't need to pay anything back to them and if they query anything about your chat a couple of weeks ago, make sure as others have said that you thought you were discussion possibilities and options rather actually doing anything.
have you had anything from them in the post with tax forms and final payslips or anything else incriminating?
Or any follow up notes from the meeting to confirm what was said?
Could you write a letter or email now to them, just as a polite follow up to the meeting, to say thank you for the meeting and for outlining the different possibilities once your maternity leave is over. Ask them to confirm in writing when your expected return date is and any other dates of note by which time you need to tell them anything.
If you can get this to them tonight - or at least before you ring them tomorrow - if nobody else has written anything else down about it then at least yours will be the written record of the meeting. Definitely write it as if you don't know what your brother has told you and as if you are expecting to go back (at least for a short while!) but expecting them to provide further info for clarification.
Then when you speak to them you can express surprise if they say 'oh but you don't work here any more' and ask if you've been fired or done something wrong. Then they'll have to tell you that you have resigned - so you can say something along the lines of 'I haven't resigned - when am I supposed to have resigned - I think I would have known if I'd resigned. arrrrghh' And you can then refer to your email to say 'why would I have written to you asking for clarification of return dates and so forth if I'd resigned? And I have't written to you to resign - is somebody playing a really bad joke on me'
If they refer to the conversation with your line manager definitely make sure that you say that you never said you were going to resign - but don't actively mention this until they are the ones talking about you having resigned.
Depends on how well you get on with your line manager and how much you want to dob them in it but the phrase 'seems like we must have been talking at complete cross purposes' can be a useful one to get out of situations like this without directing blame at yourself or them.
Make sure you watch your bank to check pay goes in as you are expecting it to - both time and amount.
But definitely one to be sorting out tomorrow, even if you do find yourself having to go into work for a few weeks in order to get it all sorted come your return...
good luck, hope you get it all sorted and let us know how you do!