Reason behind the feud between WWE's "Wyatt" family members

Last televised match of the trio - September 19 Smackdown
Beginning of "breakup" videos - September 29

Wyatt return - Hell in a Cell, October 26
Rowan return - October 31 Smackdown
harper return - November 10 RAW
Episode of faction joining - November 17 RAW.
Reason? Too early to say. Rumours are Vince didn't like them as a group.

Long Answer:

That's surprising, because both are monsters, how can one participating in noble cause with team cena, against his brother.

Welcome to professional wrestling. Wrestlers change factions all the time. Take someone like Kane, who has gone back and forth between good and bad multiple times in his career. He's just one example, but there's a very large number you can find by a few simple google searches.

Does he want to help cena or want to take hands on his brother (due to something that I don't know and asked here) how did this happen and in which episode?

It's not clear exactly which episode you want information in. I'm assuming you're asking in which episode did the final turn occur in, in which case the answer is the November 17 episode of RAW. In it, Luke Harper won the Intercontinental Championship, whilst Rowan was revealed as a member of Cena's team, creating a feud between them. A list of all their turns can be seen on their Wikipedia page.

As for what their motives are... who knows? It doesn't seem to have been decided yet. Rumours are always rife in the WWE and they often leak "fake" rumours which can be used to help develop feuds. Daily Wrestling News reported back on the 7th October, 2014 that Vince McMahon was apparently not their biggest fan:

One source pointed to the fact the writing was on the wall for the faction and that Vince was essentially killing them off so why not let him have his way and push Harper. This same source expressed concern because Harper is nowhere close to Wyatt on the mic.

Another issue is that with the talent roster so thin and Seth Rollins and Dean Ambrose working towards the top of the card, Vince feels that by splitting The Wyatts, they can end up with two top acts instead of one.

Sean Meyers, a writer for RantSports, speculated:

Just like all three members of The Shield excelled after their split, the Wyatt Family can achieve the same success. Bray Wyatt is destined to be in the main-event picture for years to come, but the outlook for Harper and Rowan could largely be dictated by their roles at Survivor Series.

This obviously suggests that in his view their roles and reasons for their feud will largely be dictated by how well received they are in their Survivor Series fight.

Ultimately, this isn't unusual. Wrestling is fairly dynamic and ratings dictate storylines. If the fans enjoy the role of both brothers on opposite sides, they'll continue with it. If not, it'll disappear before you know it.