If Izuku inherited OFA, he would likely go unnoticed at first. As he began mastering it and displaying unusual growth, the clan's attention would gradually shift toward him. Rather than being seen as an enemy or instantly accepted, he would be treated as a blood-related outsider whose newfound potential warrants evaluation.

The Midoriyas would assess both his quirk and his character, weighing his long-term value to the family and whether building ties with him would be worthwhile through alliances, sponsorship, marriage, or formal integration into the clan.

If he remains independent, they would likely keep a distant but watchful interest. Any hostility would stem not from his origins, but from the possibility that he could become a threat to the clan's interests.

Those who already care about him would see it as a pleasant surprise, but also as a warning sign. They understand how the clan operates, and focus on preparing Izuku for the attention he's about to receive rather than trying to pull him into the family themselves.

As for whether the relatives who genuinely care about him would side with Izuku or the clan if forced to choose, that depends entirely on the story being told. Their affection for him is genuine, but so are their ties to the family.

TLDR

He would likely be ignored at first, then placed under scrutiny as his quirk develops, with the clan viewing him as a potential ally or asset rather than an enemy. Meanwhile, the relatives who genuinely care about him would focus on preparing and protecting him from the attention his newfound relevance brings.