Former Toulouse Olympique forward Hepi, 28, has plenty of Championship experience given his four years with the French club.
He has struggled for game-time at Wheldon Road this term but did feature in their Challenge Cup semi-final win over Warrington Wolves and is looking forward to making his mark with York.
Along with the fellow new arrivals, Hepi is likely to debut at home to Batley Bulldogs on Sunday.
City Knights coach James Ford said: “I’m absolutely delighted to bring Tyla to the club.
“He is a player I’ve admired for a long time.
“He’s a player with a lot of aggression and tenacity and I’m looking forward to seeing the impact he will have on our team and performances.”
New Zealander Hepi said: “I’d like to say a big thank you to York and all involved for making it happen.
“For my time here I’m looking to get some well-needed game time and playing under fatigue with experienced players.
“I know York have an abundance of knowledgeable players who have played at the top level for some time.
“Hopefully I can learn from them and James Ford and also add value to the team through enthusiasm and tenacity.”
Ex-Hull FC, Castleford, Hull KR and Huddersfield Giants half-back Ellis has joined York to ease their creative headache and his deal will be reviewed week by week.
Already without Morgan Smith, who is recovering from his pre-season ACL injury, City Knights have also now lost their Australian stand-off Brendan O’Hagan after he was injured against Halifax Panthers on Sunday.
“I’m really pleased to be in a position to bring a player of Jamie’s experience and ability to the club," said Ford, about the 31-year-old.
"I’m really looking forward to working with him and seeing the impact he will have on our team.
Featherstone's Sweeting, 21, can full-back, half-back or at hooker and joins Knights to provide competition and cover after Kriss Brining also sustained an injury against Halifax.
York winger Perry Whiteley has gone the other way to Post Office Road and Ford said: "I've seen Jake play for Castleford against Featherstone and Featherstone against Hunslet and he caught my eye in both games.
"I'm looking forward to working with Jake."