Is it okay to date my grandfathers girlfriend's grandson?

Okay so my grandfather isn't related to me by blood; he's my dad's step-dad who dated my grandmother from the time my dad was like 10? Then they got married in like 2007 or something. I've always seen him as my grandfather though and my dad has seen him like a dad. However, my grandmother passed away in 2015 so my grandad began dating women but he met this lady in 2017 and they've gone on multiple dates and have been dating for like a year or two and she treats me like family. HOWEVER, I have a crush on her grandson who is like maybe 3 to 5 years older than me? (I'm 18f). Anyway, I just want to know if it would be acceptable, even if my grandfather and his grandmother end up getting married, or should I just ignore it? I'm worried about what my family would think because their opinion means more to me than the world but I want other peoples opinions: I haven't told anyone about it yet.