It took me a few tries to get these two, especially Interpersonal Diplomacy. They pretty much go hand-in-hand so it's less of a hassle to go for both in one run. If you follow what I did you should be able to get both achievements no problem. You can follow it loosely or closely, but either way it's important to choose Elodie's moods and skills carefully. In particular if your overall Conversation skills are not high enough you are going to have problems getting these achievements.

Also note that this guide will NOT take you through the Old Forest, instead you'll talk to Briony about her family and afterwards you can continue playing however you like. If you don't see an option to talk to her about her parents then something went wrong.

Additionally, if you haven't gotten the A Timely Word achievement yet, you should get it as you go through this guide (when choosing to speak to Adair's father at the gala).

Here are the skills Elodie will need for these achievements.
They are listed more or less in order that she will need them as the story progresses.
Conversation in particular must be high.

Reflexes + Archery
to make it safely to Gwenelle's party

Lore and/or Novan History (I chose to only max out Lore and not bother with History)
to unlock the option to offer to help Briony

Conversation
to unlock the option to ask Briony about her parents for the Interpersonal Diplomacy achievement

Ciphering
to read Briony's letter and understand its meaning for the Better Left Unsaid achievement

Week 0
Menu: [Tell me more] [I've played this before]
Player chose: I've played this before

Week 1
(Studied Meditation in the morning.)
(Studied Meditation in the afternoon.)
Play with toys.

Week 2
(Studied Lore in the morning.)
(Studied Lore in the afternoon.)
Menu: [Send her away] [Arrest her] [Let her stay]
Player chose: Arrest her
Visit the dungeon.
Menu: [Hear her out] [Taunt her]
Player chose: Taunt her

Week 3
(Studied Divination in the morning.)
(Studied Divination in the afternoon.)
Talk to your father.

Week 4
(Studied Meditation in the morning.)
You have unlocked a new outfit!
(Studied Divination in the afternoon.)
Talk to your father.

Week 5
(Studied Divination in the morning.)
(Studied Divination in the afternoon.)
Visit tomb.

Week 6
(Studied Reflexes in the morning.)
(Studied Reflexes in the afternoon.)
Menu:
Player chose: I won't free her
Attend service.

Week 7
(Studied Flexibility in the morning.)
(Studied Flexibility in the afternoon.)
Menu: [Free Duchess Julianna] [Give her title to Ignatius]
Player chose: Give her title to Ignatius
Play with toys.

Week 8
(Studied Dance in the morning.)
(Studied Dance in the afternoon.)
You have unlocked a new outfit!
Play sports.

Week 9
(Studied Reflexes in the morning.)
(Studied Reflexes in the afternoon.)
Attend service.

Week 10
(Studied Reflexes in the morning.)
(Studied Reflexes in the afternoon.)
Menu: [Surrender Province] [Offer him money if Ixion withdraws] [Bluff / Intimidate] [Execute him]
Player chose: Offer him money if Ixion withdraws
Menu: [Agree] [Refuse]
Player chose: Agree
Sneak out.

Week 11
(Studied Public Speaking in the morning.)
(Studied Public Speaking in the afternoon.)
Visit the dungeon.
Menu: [Taunt her anyway] [Execute her] [Leave her alone.]
Player chose: Leave her alone.

Week 12
(Studied Lore in the morning.)
(Studied Lore in the afternoon.)
Menu: [Accept his offer] [Politely decline]
Player chose: Accept his offer
Play with toys.

Week 13
(Studied Court Manners in the morning.)
(Studied Lore in the afternoon.)
You have nothing more to learn about Lore.
Menu: [Put her to work] [Imprison her] [Execute her]
Player chose: Execute her
Walk in the gardens.

Week 14
(Studied Court Manners in the morning.)
(Studied Flattery in the afternoon.)
Attend service.

Week 15
(Studied Flattery in the morning.)
You have unlocked a new outfit!
(Studied Public Speaking in the afternoon.)
Walk in the gardens.

Week 16
(Studied Public Speaking in the morning.)
(Studied Public Speaking in the afternoon.)
Menu:
Player chose: I would rather not go
Menu: [Confess your fears] [Yell at her] [Make religious excuse]
Player chose: Make religious excuse
Visit the dungeon.

Week 17
(Studied Court Manners in the morning.)
(Studied Flattery in the afternoon.)
Menu: [Dance with suitors] [Refuse]
Player chose: Refuse
Menu: [Talk to him] [Say nothing]
Player chose: Talk to him
Sneak out.

Week 18
(Studied Court Manners in the morning.)
(Studied Flattery in the afternoon.)
Menu: [Shame her with silent scorn] [Insult her with false flattery] [Trip her as she passes]
Player chose: Shame her with silent scorn
Attend court or Play sports.

Week 19
(Studied Archery in the morning.)
(Studied Archery in the afternoon.)
Menu: [Raise taxes] [Keep them the same] [Lower taxes]
Player chose: Raise taxes
Attend court or Play sports.

Week 20
(Studied Archery in the morning.)
(Studied Archery in the afternoon.)
Menu: [Pardon him] [Imprison him] [Execute him]
Player chose: Imprison him
Sneak out.

Week 21
(Studied Swords in the morning.)
(Studied Swords in the afternoon.)
Attend court.

Week 22
(Studied Public Speaking in the morning.)
You have nothing more to learn about Public Speaking.
(Studied Reflexes in the afternoon.)
You have nothing more to learn about Reflexes.
Menu: [Leave him with Arisse] [Send him to his grandfather] [Send him to your uncle] [Make his stepsister Adele regent]
Player chose: Make his stepsister Adele regent
Sneak out.

Week 23
(Studied Polearms in the morning.)
(Studied Polearms in the afternoon.)
Menu: [Accept her as Court Musician] [Reject her]
Player chose: Reject her
Sneak out.

Week 24
(Studied Swords in the morning.)
(Studied Archery in the afternoon.)
You have unlocked a new outfit!
Sneak out.

Week 25
(Studied Archery in the morning.)
(Studied Archery in the afternoon.)
Menu: [Hire more soldiers] [Do nothing]
Player chose: Hire more soldiers
Sneak out.

Week 26
(Studied Archery in the morning.)
(Studied Archery in the afternoon.)
You have nothing more to learn about Archery.
Menu: [That's hilarious!] [That's terrible!]
Player chose: That's terrible!
Menu: [Climb out and grab it] [Ignore it]
Player chose: Ignore it
Attend court.

Week 27
(Studied Court Manners in the morning.)
You have nothing more to learn about Court Manners.
(Studied Flattery in the afternoon.)
You have nothing more to learn about Flattery.
Menu: [Go to Sudbury for Gwenelle's party] [Send your regrets]
Player chose: Go to Sudbury for Gwenelle's party
Play sports.

Week 28
(Studied Novan History in the morning.)
(Studied Novan History in the afternoon.)
Menu: [Side with Gwenelle] [Side with her mother] [Flatter them both]
Player chose: Side with Gwenelle
Menu: [Offer to help] [Try to talk her out of it] [Tattle to her parents]
Player chose: Offer to help
Play sports.

Week 29
(Studied Strategy in the morning.)
(Studied Strategy in the afternoon.)
Menu: [Go with Briony] [Send her alone] [Tell her not to go] [Ask about Briony's parents]
Player chose: Ask about Briony's parents
Play sports.

Week 30
(Studied Ciphering in the morning.)
(Studied Ciphering in the afternoon.)
Visit the dungeon.

Week 31
(Studied Ciphering in the morning.)
(Studied Ciphering in the afternoon.)
Visit the dungeon.

After you cipher the letter and get the achievement you can continue playing however you like. Hope this helped!