Translation taken from here. Credit goes to Zysora & the FGO Facebook community.
Another translation that I was slightly skeptical about, so I did some minor adjustments from the original translation. Another proof-reading would be appreciated if I did mess up the translation.
There's nothing exactly wrong, but there's some context missing that is what makes the joke...
Oh, and when you can, Danbooru translation policy is to keep honorifics. That means 初代様 is "Founder-sama". (様 = sama, さん = san, くん = kun, and ちゃん = chan.)
I like the little detail of Cursed Arm's parka only having one sleeve, makes you wonder how he got it on though.
You didn't do anything funny with them right...?Big SisMotherSince the cold season is here, both of you should wear these Parkas!As I thought!Cursed Arm, too...Little sisMother...?Great Founder!!If that's the case, then the Great FounderThank you so much!And he didn't notice the writing on the back...The Old ManThis is the same character as in his title, "The Old Man of the Mountain."will have "father" or "grandfather" on his!Heeere you go!*Clatter!*