This is iphone 7 plus silicone case fake?

Might be. If you don’t like it, return it. Apple hasn’t made or sold silicone cases for the iPhone 7 in a very long time. I’d be surprised if you could actually find a genuine case any more.

It certainly looks authentic. But whether it is or isn't, only matters if there is an issue with the case. As long as it fits you phone well and you like it is the only thing that matters. As KiltedTime correctly suggests, iPhone 7 is a very old iPhone at this point. It was released in 2016 and discontinued in 2017. It's rather remarkable you managed to find a brand new Apple Case for a phone which was discontinued nearly 8 years ago.

Looks real. It’s hard to say. It’s been a long time since I had an iPhone 7.