How do I approach this?

[Whereamigoingwiththis]

How can I confirm or discard this information in relation to my family tree?

My parents marriage appears to be the only one with their combination of forenames and surnames in the bmd register. One of their forenames and surnames is extremely uncommon in this area.

There is a birth record of a child born to parents with the same surname/maiden name as my parents, born in the same city as me and my siblings within the time frame of their marriage.

I suppose I am asking if this could be a coincidence, a mistake in the records or even a sibling that was subsequently adopted? And how do I find out which one it is without subscribing to a genealogy website. Parents are no longer alive and their remaining direct family are not able to answer questions.

You could order the birth certificate. Is there a death registered?

Totally agree- order the birth certificate. That would be my first step, anyway.

Gosh OP. Gently, the first thing that would occur to me is that the child died in infancy. It isn't so very long ago that if such a thing happened a fairly common response would be to never talk about them again (I actually know a family who did this, later children had no idea there was a deceased sibling until the information emerged when they were mature adults). I am NOT any kind of expert in this but a mistake seems unlikely and if your parents were already married why would the child have been adopted?
I suppose the other possibility is a disability of the kind that would, in past times, have resulted in the child being institutionalised?
I'm so sorry this must be very shocking. I hope you can get further information.

I thought you need the full date of birth to order a birth certificate, bmd register only gives the quarter in which the birth was.

That’s all you need to order from here

https://www.gov.uk/order-copy-birth-death-marriage-certificate

Will double check to see if the child died, but I'm sure I checked that at the time I discovered it.

You're right about child death or disability not being talked about though.

You can order from the GRO website with the quarter and registration district, together with the names you have. Don't order from other websites offering a similar service, they often rip you off with higher charges.

I agree with others about searching for a death record. My late DM never told me she had three brothers - I only knew of my one uncle. The other two died as small children and for reasons best known to herself, DM did not tell me of their existence. I only found out after her death, when I started doing family tree research.

Forgot to mention - don't narrow down the search too much - sometimes registration district boundaries change, or the family was at a different address for a time. Do a wider search for a death, using just the approximate year of birth, and for a good few years following that date.

Another option would be that if the family lived in the same location for many years, there may be a possibility that there are parish registers for local places of worship that you could check for family records. If your parents married in a church, that would be the place to start.

No death registered for the child's name within 12 years of the birth anywhere

@Whereamigoingwiththis Okay, have you looked at all your other relatives, uncles, distant cousins and so on? Find birth records for all of them. I've sometimes found that going sideways helps. You can even get useful information about people who turn up named as witnesses on other people's marriage certificates.

If you don't want to pay to subscribe to a website OP you can use the free Ancestry Library edition, available online at all UK public libraries.
That allows you to access census records and a lot more, though you still have to pay the GRO to get birth certificates etc.

There are no uncles on my dad's side of the family so going sideways didn't help.
Parents got married in a different county, so that doesn't help either, they moved back here after they got married but before my eldest sibling was born.
Mum's relatives lived in a different county altogether.

Are there any aunts on your dad's side of the family?

What decade was this potential sibling born?

Yes there is an aunt on my dad's side. Decade in question is the 60's.

I did a free search on ancestry but drew a blank, that doesn't surprise me because I couldn't find anything on my grandad either, which is why I didn't want to subscribe again

You may be able to get a digital image immediately from gro.gov.uk

You can get quite a lot of free information from FreeBMD, FreeCEN, FreeReg. The first one is births, marriages, deaths since compulsory registration was introduced and is complete up until relatively recent times, the other two are census and parish records and are more patchy so they don't include all the records, but they might include something you want. However it doesn't sound like you are looking to go back preregistration at the moment.

Thanks, I didn't know about FreeCEN or FreeReg.
I had already traced my mum's family back to the early 1700's mainly through an ancestry subscription and parish records and also some very helpful gents on WW2talk.
After a sketchy start in the search into my dad's side of the family I've now gone as far as I can at the moment with that too.

That's when I decided to take a look at this particular mystery.

What do the dates look like in terms of birth order? Is this child the oldest / youngest or do dates conflict with their known children?

I think you’ll have to order the birth certificate to find out if it’s just coincidence.

The child would have been the youngest if it wasn't a coincidence.

I regret sitting on the information for so long, and I suspect that you're right and I will have to order a birth certificate but even that will have to wait a bit longer.

@Whereamigoingwiththis if you find the birth certificate on the government website it shows the mothers maiden name before you order fully and pay if that would help at all.

I've searched for deaths on there by going through a load of years. Took a while but got some answers.

For a birth in the 1960s you won't be able to get an instant online digital image, unfortunately, or even a pdf. You'll need to pay for a certificate, which costs £11.

Details here:
https://www.gro.gov.uk/gro/content/certificates/faq.asp