I want to buy my sister a laptop, but I do not know which. She wants to play mainly Sims 4, the budget is just under 300 euro
Not for the budget. Lappis are already more expensive than PCs and then only 300 euro? There are used on ebay.
MacBook
And which MacBook does one get for 300 euro? Not even on the current models for 1200 euro, Sims will run really well.
For 300 euro you will not find a laptop on which Sims will run useful. Since you get just a very simple office laptop for Word and the Internet. For any games, however, unsuitable.
At least 600 euro you'll have to take in hand, and then preferably a model with a Ryzen 5 processor (for example, Ryzen 5 3500U). They have relatively good graphics units, because Sims could run halfway on it.