To be honest, Shirou's son/daughter and Rin's daughter/son could marry and their children would still call them grandpa and grandma(on the father side and on the mother side).
But of course the first thing you think of would be this kind.
There's a continuation where the Guda's show a picture of Ishtar and Rin gets embarrassed and tries to get Shirou to not look
With how this is going, I could say it—What the heck's wrong with you guys!!H—What are you—
No way. No way!Which means these two are Rin and Shirou's......Grandma.HUH!?Come to think of it, he does look somehow similar to Toosaka......You got it wronnnnnggggg!
He doesn't!Errrm. Are they siblings......?Toosaka's and my...!?