I recently had an idea of creating a "cut scene" of sorts where the BBEG's ghost sister shows the PC's a past event, but I also want to include my players so I'm not ranting for ten minutes straight. The reason why I wanted to come up with a "cut scene" is because I also don't want the BBEG's sister to rant either. I guess I'm ranting either way, but are there any tips or other ways to go about this? Anything helps, thanks!

Maybe go the A Christmas Carol route. The sister shows up and guides them through this event in a dream or as psychic projections. They can all speak to one another, (and ask questions of the sister) but they can’t interact with or change the scene.

Yeah if the event truly takes 10 minutes to communicate, I like Xalthu's suggestion. But if you tighten up the wording and realize you can get across all the important stuff in under a minute, it's fine for you to just take the reigns for that long.

There are a few options other than what was mentioned 

• The player can take the place of characters using their stat blocks or others to experience the events. Even if they behave differently than what you intended for the story and you don't want to change it you can just finish off by saying " you were victorious but they were not..."  or something similar. 
• You can break it up into pieces.  For example they could be taken to a location and have to look for evidence. When they find pieces they get brief visions.
• Something I've done in the past is simply write it out as a vision they're given before the session. I often do this for dream messages, diary entries and alike.
• You can also find less dialogue heavy ways to give the information. For example by breaking it up into clues  that they need to piece the story together with. 

I would say, simply: don't. This gives the game a video-game-like quality (although you might want to do that) and removes the ability for players to do interesting things (they generally tend to like doing things and being the centers of attention). I would say, have the scene play out, and at certain intervals, ask the players: "do you want to do anything?" This will make it interesting for them and you.

I needed to edit because I forgot to read the top post. Yeah, still allow them to do interesting things, just don't leave them out of it in whatever way possible.

If the past events you're depicting in the cutscene involve combat, you could consider placing your players *in* the scene having to essentially act out the events. They find themselves in a memory and the npc's they interact with don't see them as themselves but as the figures in the scene. If combat happens, the players have to fight through it (either with pre-gen characters handed out for the occasion or just as themselves but with the same caveat that others simply see them as the original figures in the scene).

I've made this work with flashbacks before and players usually find it a fun way to get the download on some lore while still being involved. Also it gives you a chance to try some truly broken encounters because when I ran it, it was the memory of an order of warriors failing to protect an ancient solar from a demon worshipping cult, so not only is failure painted into the scene, but it's only the memory of a failure so a TPK doesn't actually effect the player characters. It's a great way to kind of stress-test the survivability of your party to be able to prepare more challenging encounters that ride that line between do-able and deadly really well. 

If still relevant, I'd recommend looking at the Decent Into Avernus adventure. 

Minor spoiler alert for the book: 

At one point in the adventure, the players are taken through a series of "dreams" of past events where they witness the downfall of a certain notable fallen angel. I'd highly recommend checking it out, as this seems like exactly what you're looking for and also falls in line with what the rest of the commenters have been saying. 

I've done a "while this character was away, this is what they did and here's who they met" session that was presented as a flashback of sorts. I realized I could give them a big piece of world building and tie it in to why this character wasn't around for a couple sessions. 

My players were down for a change up so I handed out some stat blocks and a couple of secrets they had that one of our players had to figure out. They played as the characters he interacted with while his character was "away" from the group, but really the player just missed a few sessions. 

To bring this back around, your bbeg's sister does a wavy hand crossfade to show your players (and their characters) a scene in the past. You hand out a couple of character cards of key NPCs in this story to your players with a couple of crucial bits of information, but no one person has all the clues. Then you as the ghost sister guide them thru the past event and help them piece it all together as a team.

The only tip I've ever seen on this was to avoid doing it.  Show, don't tell is very common advice, and a cut scene is the very opposite of that:  An information dump where you just tell the players something rather than letting them experience the thing.

Not sure if this has been suggested, but Descent into Avernus has the characters basically play through dreamlike scenes that are relevant to the BBEG's backstory.

 Not sure this is worth a spoiler but better safe than sorry