I’m just wanted to comment as my other half if in a similar situation with his brother so may help.
My brother in laws son was born with a chromosome abnormality and has a few minor deformities and delayed development. When he was born they were offered genetic testing on both parents to check if this had been passed on, this was refused by them at the time which we thought was odd and we never heard anything of it since.
As his son has aged his skin has become increasingly darker to medium/olive tone. My brother in law does have tanned skin for a white man, just like my other half, but our children and rather pale in comparison which really shows when they are together. His mum is white but slightly darker than myself as I am rather pale but not drastically. We have never said anything as we are not sure if this could potentially have something to do with his chromosome abnormality (as we don't know much about it )and have had no reason to not trust his girlfriend before.. until recently when he caught his girlfriend, who was early pregnant at the time, with another man in their home. He obviously questioned the paternity of the unborn child but she swore it was his. He refused to believe her and said he didn't want anything to do with the baby until she had a dna test, so she has continued the relationship with this new man and terminated the pregnancy.
At the time brother in law asked my other half if he thought the first born was his, and he just advised him to get a DNA test. When my other half next spoke with him and he asked if he was going to get a test he just said he knew his son was his and changed the subject.
My other half doesn’t know what to do anymore as he feels he cannot bring it up again but is sure others in his brothers close network (they don’t have any family left) must be thinking the same and wondering why he as his brother hasn’t said anything.
It’s a really tricky situation but like others have said, it's difficult to take this questioning back and could cause a massive fall out if we are wrong/they do not want to hear it. My brother in law obviously doesn’t want to believe it or deal with it, and that’s his decision to make, we can only be there for them if this does indeed turn out to be true.