Eli Apple's brother says it's the NY Giants cornerback's MOTHER who's the real cancer in his life -- and he's begging his bro to cut her out before she kills his career.

TMZ Sports spoke with Dane Blackson -- Eli's half-brother (they have the same mom) -- and he's very passionate that Annie Apple is "pulling the strings" in a very damaging way.

"I feel like when people say Eli's like a cancer ... I just think of mom when people say like words and use these types of adjectives."

"I feel like he never had a fair shot. I mean, a lot of this sh*t that you could be judging him on, there's really a master behind him pulling the strings. So, as far as Eli being the cancer, I don't really see Eli being the cancer more than I see mom being the cancer and him not being able to distance himself."

Annie Apple has had a major presence in Eli's life ever since he broke out as a star college player at Ohio State.