[RPG] roll to maintain concentration if a target other than me who is affected by the concentration spell takes damage

Since the question on whether or not a sprite can remain invisible and use its help action has already been answered, I'll jump to the one bit that hasn't been answered so far:

"My only concern is: is such help strong enough to provide me with advantage?"

Yes you get advantage

The Help action itself is quite remarkable (and underused in my experience): you give up your attack - and thus your ability to do damage yourself - in order to give one of your allies advantage on its next attack on one specific creature you choose.

You give up your Sprite's damage to the foe and in return you get a greater chance to hit, and thus have a bigger chance of doing damage yourself.

The way your sprite's Help action plays out is mostly flavour. What is does, however, is determined by the Help action's definition, which you've already found.

My answer to your question would be: Yes the Sprite would grant you advantage.

But...

You have already stated you're aware of the fact that creatures could sense the fact that your sprite is standing/flying next to them, when you wrote: "(but of course can still be sensed by some creatures, or heard by them)"

Keep in mind that your Sprite has to be within five feet of the enemy in order to take the Help action and can still be attacked while invisible, although this means the attacker will do so with disadvantage, due to the way the Invisible condition is described:

Invisible

An invisible creature is impossible to see without the aid of magic or a Special sense. For the purpose of Hiding, the creature is heavily obscured. The creature’s location can be detected by any noise it makes or any tracks it leaves.

Attack rolls against the creature have disadvantage, and the creature’s Attack rolls have advantage.

If your sprite were to stay in the enemy's reach, this means they could attack it on their turn, but with disadvantage. Your Sprite can, however, fly out of the enemy's reach without provoking an attack of opportunity, assuming it has any movement left.

RAW-wise, an attack of opportunity can only be made against a creature the enemy can see, which gives your Sprite the possibility to aid and move out of the enemy's reach without taking any damage and without requiring the owl's "Flyby" feature.

Edit: I found out this question has already been answered, for more elaborate explanations, go here.

The rules, as written, don't say.

There are no specific rules regarding the contents of an invisible mage hand but there is evidence that objects held in the hand may be invisible.

While we don't have many general rules regarding invisibility (mainly just what's in the "Hidden" sidebar in the PHB), we do have an invisibility spell and an invisibility monster action to base our decision off of. We also have usability to consider; why would the designers give us the ability to turn the mage hand invisible if it gives away its position as soon as it picks something (anything) up?

Invisibility the spell says that all objects worn or carried by the target are invisible. This seems fairly harmless and renders the spell kind of useless if it doesn't give completely invisibility to all objects carried and worn.

Under the control of a skilled arcane trickster, the mage hand can be used to manipulate objects, pick locks, pick pockets, or carry an object or objects weighing up to 10 lbs. It can also turn invisible, per the arcane trickster's class feature.

With that in mind, we can probably make some assumptions about the hand:

• It is probably the size of an average person's hand
• It is dexterous, or at least as dexterous as its controller
• It can manipulate small objects such as a lockpick
• It can steal things from people

We also know the following:

• In other cases in the rules, invisibility turns held objects invisible. This alone is not reason enough, but because it makes the spell invisibility useful (otherwise enemies would see your sword and attack you sight unseen) it is important to the argument (usability, as I mentioned above).
• The arcane trickster is a class archetype based around a rogue who uses magic to play tricks and get away with his roguish activities.
• The arcane trickster gets special rules for his mage hand.

Taking these things into consideration, it seems quite in line with the design goals of 5e and the overall flavor of the class to allow objects held in the hand and completely obscured by it to become invisible. This includes as many coins as can fit inside the closed fist, a dart, dice, a key, lockpicks, or other small, "palmable" objects. Larger objects, such as weapons, planks of wood, mugs of ale, and so on, would be visible because they are not completely obscured by the mage hand.

My reasoning for this:

• Invisibility the spell and invisibility the monster action generally grant invisibility to the target and all items worn or held.
• The arcane trickster relies on subterfuge and trickery. Why give them an invisible mage hand if it can't hide anything inside it? That's like saying, "Here's a beer, but don't drink it!" Or maybe more like, "Have this beer, you can drink it, but only when no one is looking. Oh yes and you're in a crowded marketplace."
• For the skeptical DM: It doesn't hurt anything to allow this! In fact, it will probably make your games more fun when your Arcane Trickster's player is having more fun.
• It isn't broken. It doesn't imbalance anything in the gameplay and it's easily overcome or made up for in other areas by a good DM. Personally, I don't think it even needs to be "made up" for. It is by no means a showstopper.