does it count as incest since they are sister ships?

I've never been entirely clear on how that works (I suspect the concept of biological sisters isn't really relevant in the first place for kanmusu), but for tagging purposes I'd say no - I've never seen it on Kancolle stuff in the past, in any case.

Ships would make for some...interesting family trees, though - a Fletcher-class DD would have 174 sisters and 157 of what I suppose would be half-sisters (Sumner/Gearing classes).

Edit: While we're at it, a LST(2) tank landing ship would apparently have 1037 sisters, and if you want to go as small as the LCVP, something more like 20000. Better get some more paper.

That's a lot of sisters how are you going to remember them all?