So my sister wants to play as a witch in 5e D&D

She specifically wants to talk to the spirits of the undead though. Are there any sort of class features that would allow her to do this from early on?

no

Male witches are called WARLOCKS! Therefore, a female warlock is a witch. There is a warlock class in the game. Problem solved.

Ok, I guess I'll just use a reskinned warlock then.

is speak with dead a ritual? if so, have her start with that and the ritual caster feat or whatever it is. It shouldn't be to high a spell, and hell she can probably make a few gold helping people get closure with loved ones. For potion making you'll have to improve a bit, but you could make a list of items needed for each one, and the time it takes to brew it fairly easily

Regretably, speak with dead is not a ritual.
For some reason.

As other posters have said, this is indeed a very clear cut case of a druid. However, it's neither swamp, nor underdark one. Circle of twilight is a much closer fit.
astranauta.github.io/classes.html#Druid,Circle of Twilight (UA)

Here, take a look.

>I'm not sure she's taking this seriously
Yeah fuck all those plebs who think we're just playing games when we get together to roll dice and pretend to be magic men throwing fireballs. You should probably have bandits rape her character in the first session, that will teach her that /tg isn't fun and games for babies.

Make it one then? Have it take extra to cast as a ritual or something to signify that its slightly different than other rituals. Or make it just a normal one if you dont think its a big deal for a one shot. Conversely you can just add it to her spell list if it isn't already part of whatever caster class you go with? Seeing as it is a oneshot its can be as complex as you wanna make it.

Maybe a shaman? She could play as a witch doctor

There's no such thing as a shaman class.

Warlock with that one UA invocation that lets you ask questions of the dead for your magic book? You get a sweet pet bird too.

user, I think you might be overthinking this a little. Especially with concerns like this Why do you need a class feature? Just let her do the thing. If you're that worried about it, there's a Background in the Curse of Strahd options doc that works perfectly: the Haunted One. One of the Flaw options is "I talk to spirits that no one else can see."

Bam. There you go. She can talk to spirits. Use it in the game for fun

Knock it off you contrarian prick. There's a difference between people having a good time playing a game and "not taking it seriously" and someone who is mocking the game and players and "not taking it seriously" I'm not surprised you can't see the difference between the two, unable to pick up social cues is a symptom of autism, after all.

Feylock of the Chain
OR
Swamp Druid

Pathfinder has an actual witch class

Alternatively, some kind of sorcerer

As much as I enjoy 5e (current system of choice), you're going to have to explain to her that she won't get exactly what she wants out of her character. I'm assuming that this is her first tabletop rpg (because her character is pretty much the same as what my sister wanted to play her first time), so the problem I'm seeing is less "5e isn't a very malleable system" and more "first time roleplayers usually aren't keen on making sacrifices to their vision of their character in order to fit the character into the system/setting." Reading the thread, it looks like you have a few options that will probably work, but I'm willing to bet that she'll want some things that either you can't give her within the guidelines of the rules, or things that she won't like your solution to because they don't fit the feeling she wants. I'm not saying that you should switch systems, but you'll want to make sure that she understands that not everything she wants out of her character will come to fruition simply because you have to worry about balance. It sounds like she's pretty creative, but until she learns the rules of the system and keeps them in mind during her concept design for characters, she'll be disappointed in the things that she simply can't do due to balance, and conflict with 5e's rule set.

Then again, maybe she's an experienced roleplayer who is used to dealing with crunchy systems and I'm just talking out of my ass. Feel free to ignore me if this is the case.

Wrong system. OP said 5e

Druid or Cleric. Not sure why you have any problem desu. Swap some Druid/Cleric stuff with some Wizard/Sorcerer stuff if it doesn't feel quite right just refluffing.

Witch is a title applied to a person who does magic, in the modern world it has certain pop-culture connotations (except when it doesn't).

All the classes that can do magic can qualify as a witch.

Fuck dude, this shit isn't hard to figure out.