"I’ve found a birth in 1935 which I think is him."
You can order a birth certificate from the General Register Office to confirm this. @ExOptimist mentioned that a digital image is £3 but that is only available for birth certificates up to 1926. For any birth from 1927 onwards you need to pay £12.50 for a paper certificate.
"...anything connecting him to the 2 sisters or indeed the father"
I would suggest, instead of looking for him in the 1939 Register, have a look for the mother.
Many records of people on that Register born in the 1930s are still redacted. If you have a look at the actual register on Ancestry you will see that the entry has been edited out and says "This record is officially closed".
Records are only unredacted after they reach the age of 100 or there is notification of a person's death. In can be a bit hit and miss though. For example, I have a relative who was born in 1937 and died in 2011 and their name is viewable on the Register. However, they had a brother who was born in 1936 and died in 2023 and his name is still redacted.
So, rather than looking for the brother on the 1939 Register, have a look for the mother. Then have a look at the actual entry on the Register and see if there are any redacted entries under the mother.
If there are then that will very likely be the brother. You can't be absolutely certain but it is evidence that the mother had another child living with her at that address.
"No marriage records that I can see "
Do be aware that even if you click on Location and select "Exact to country" that doesn't work at the level of the UK but it is separately for England and Wales and Ancestry doesn't have Scottish records at all.
Could it be that he married in Wales or Scotland? In this situation it would be best not to include location and/or also look on the Scotlands People website.
If he was born in 1935 then typically he would be getting married for the first time anywhere from 1953 to 1975. It may be that the name you have been given for his wife may have been a nickname or her middle name or similar and so it won't show up on the records.
You may have to just look for his name and location.
Alternatively, it may be that they never actually married. I had this with the relative I mentioned above who was born in 1937 and died in 2011.
I was aware from speaking to family that he had married twice and had five children in total. Two with his first wife and three with his second.
I could find his second marriage but there was no record of his first marriage. I knew the names of the children and their rough ages so I was able to find their birth certificates. From there I was able to get the surname of his first 'wife'.
Even with that though, I still cannot find a marriage certificate. This is odd as it was very rare indeed for couples not to marry in the 1960s and 1970s when they had children.