If you share DNA with them you are related.
Just how closely is very much dependant upon how much DNA is shared, not just with you but between each of them.
The lower the shared DNA the more distant it is until you find the MRCA to each of them.
Often one has a DNA Match but there is no “paperwork” to back it up, but DNA above 20cM can be regarded as reliable, then the reliability goes down until 10cM and below when it becomes questionable and a documented trail is really needed to support the link.
DNA also can show a match but if the person was the subject of an undocumented Adoption then that can cloud the matter.
In your case it would help us if you could list the cM shared with each of the four.
Thank you - I know it sounds like such a ridiculous question, but I've been going round in circles with all of this and I just wanted confirmation that Frederick Sr must have been his biological father.
Petros has found the birth reg for Frederick Jr (fanfare of trumpets), the DOB matches, and the mother is Harriett Hudson, no father named, but it must have been Frederick Sr if I have those DNA matches. Here are the cM's....
His sister No. 1: 22cM/2seg
Sister No. 2: 9cM/1seg
Brother No.1: 9cM/1seg
Brother No. 2: 17cM/1seg
In addition, I have a 360cM match with my first cousin.
And I know I need to research this DNA thing properly, so apologies.
EDITED: As I mentioned in my first post, Frederick Sr and Harriett Hudson had two children who were born in Wandsworth, but long after Frederick Jr's birth in 1868 to Harriett. Arthur was born in 1878 and Ada in 1888, so there's ten years between the siblings. Is it possible that Frederick Sr wasn't the biological father, that he met Harriett sometime after, and they had Arthur and Ada together, thereby making them Frederick Jr's half-siblings.... and is it possible that the DNA matches I have from Frederick Sr's siblings are linked to the half-siblings of Frederick Jr? The only DNA match I have with Frederick Sr is with my first cousin. The only thing making me think Frederick Sr was the father (unnamed on the birth cert) is that Harriett registered him as FREDERICK Hudson,