Manga where the MC's mother was secretly a princess from another nation, and the MC became the crown prince of that nation

Question

I remember that the MC has a younger sibling in their family. His father is a lord (from a popular family, like a duke or noble) and has a second wife (the younger sibling came from the second wife). The father always ignores the MC and his mother, and focuses always on his second wife and her son.

A part of the story is that there was a party to celebrating the younger sibling becoming the head of the MC's family. The MC and his mother were not allowed to enter the building where the party is held, the mother becomes sad. The MC then thinks of a way to make her happy, and sees a garden which they will go to.

I also remember that his mother was actually a princess from another nation, full of different demi-humans. His mother was also a dryad, or something about being part of nature. She has a dog looking sister (the second princess). The mother actually gave birth to the MC on her own, because somehow the story says that dryads can make a child on their own, without any assistance from men.

Answer 1

This is Magic Stone Gourmet：Eating Magical Power Made Me The Strongest.

From MangaUpdates:

In this world, It's only me who can get stronger by eating Magic Stones! At one point, I received an offer to have a smooth life reincarnation from the Goddess-sama... But the skill I got as a prize is Toxins Decomposition EX. Ugh, what a plain skill! Its so pitiful that even my own family makes fun of me... However, one day I realized that because of this skills effect, I can eat and gain power from Magic Stones! I've also been notified that I'm the prince from a neighboring country!? What awaits me in the castle are days of experimenting with Magic Stones and training, the perfect environment for me to become the strongest man there is... And to top it off, I received the Magic Stone of Dullahan! The story of a boy on the path to become a King while being chased by an assertive fiancee and Female Knights starts here!

At the beginning of the story, the MC -- who, judging by his appearance, and the fact that this is a manga, I'm guessing was originally a young man from modern Japan -- finds himself in a mist-filled void, and meets a goddess who tells him that the cause of his death was pitiful, so he's going to be reincarnated in a fantasy world. He's also asked to pick a capsule from a gacha machine, and receives the skill of Toxin Decomposition EX.

Shortly afterwards, a five-year-old boy wakes up from having dreamed all this, the dream apparently being a memory of how he was reincarnated five years earlier. The boy's father is a general of the largest kingdom on the continent, and the head of the Roundheart House. He also favours his second wife and her son over the MC, who is the elder of the two boys.

Later in the story, the father holds a party to introduce the MC's younger half-brother to other noble families, and announce that he'll be the next head of the Roundheart House. The MC -- who is unsurprisingly disappointed by this news -- asks his mother to take him for a stroll in the garden while all this is going on, stating that he'd be much happier surrounded by those flowers.

Later still, the MC's mother divorces the father and takes the MC to her "fatherland", a kingdom in a neighbouring country. The MC learns that his mother is the second princess of that kingdom. His grandfather, the king, subsequently appoints him as the crown prince.

In addition, the MC learns that his mother is a dryad, who can bear a child without help from the opposite sex. The page below also shows the mother's "dog looking sister" (described as a "talking cat" by the MC), the first princess of the kingdom.