is it possible or likely that hybrids in twilight are sterile? if so, what are the implications?
Can hybrids reproduce? Hybrids and reproduction. More on hybrids and reproduction.
I lean towards they can. It's of course possible they can't, plenty of examples of sterile hybrid creatures in nature, but I think it's also something we shouldn't assume right out the door or say is likelier than reproduction being possible. And it would explain those sexual attributes they didn't lose along the way.
As for the implications of sterility... nothing?
It means hybrids will never take off as their own species, they'll happen if/when humans and vampires have children together but you'll never get them breeding on their own. It would mean there will always be relatively few hybrids (especially taking into account the conditions required for successful births).
For the personal characters, it means that there will be big drama when it's discovered that Renesmee physically cannot have her own children but that'll be overshadowed by whatever else is going on in the family and... of course... the Jacob debacle.
For Nahuel, he may never know, as he has the small problem that every female member of his species is his sister or Renesmee. It's really hard to be Nahuel, guys.
And it would mean Joham wouldn't have any success in bright ideas I really hope he didn't have offscreen even though they feel exactly like the sort of ideas he would have.