One thing to keep in mind when trying to pass on an enthusiasm for mathematics to a child who is very young is to remember that the thing that will get a kid to fall in love with math is the feeling of working hard on a problem and coming up with their own solution. Do not take this away from them!
If you're talking about a problem that's just beyond the child's ability, but not getting anywhere, there are a few ways to avoid becoming frustrated. You can try to give some hints, you can try acting things out, or you can say something like, "This is a tough problem, maybe we should think about it for a while," and then move on. This can help to teach perseverence.
Please don't teach a child that math is all about worksheets with arrays of addition facts on them. Do teach them their addition facts by asking them to add things they encounter every day: "Mario has 3 lives, but if I play this level I will get 2 free lives. How many will I have after this level?"
Try to get in the habit of asking the child to explain why they think their answer is correct both when they actually are correct and when they are incorrect. Try even harder to avoid giving them clues to whether they are correct in the way you ask the question.
Act like you don't understand how to do something and do really silly things, like mis-count, so they explain to you the correct thing. When they are explaining, try to think like a genie and ask them questions to make them clarify their explanation:
CHILD: "A bench is a thing you sit on"
ADULT: "Like this chair? Is this a bench?"
CHILD: "Noooo! A bench is long."
Things you probably forgot you learned
At one point it was not obvious that 3+2 is the same as 2+3. It did not become obvious because someone pointed it out to you, but rather because you computed 3+2 and 2+3 the hard way many times. Expect a young child not to realize this, too, and give them lots of chances to compute those sums.
Once, when you were very young, to compute 5+2 you counted "1, 2, 3, 4, 5" and then "6, 7" while looking at your fingers. Preschool age children usually don't learn the strategy called counting on until they are around 5 or so. Expect this, and give them lots of chances to practice, so that they will get tired of all that counting. They will "invent" counting on
At one point, the thing that made a square a square was the fact that it looked like a square. Turn it 45 degrees and that's not a square, it's a diamond. To young children, what makes a shape is how it looks. Try taking some drinking straws cut to various lengths and making different looking triangles. A fun game is to each take matching pieces and see if you can make different triangles. It's fun if you make your triangle in secret and then reveal them together.
Building number sense
One tool that teachers of young children employ in helping their students visualize numbers is called a ten frame and it is a grid with two rows and 5 columns. To use a ten frame to visualize a number, you put that many dots in the grid, one per cell. Make several of these. Then pull one out and ask the child to tell you how many dots are there. Then ask them how they see it. Then say a different way to see it.
"There are seven dots. I see five on the top row and two more on the bottom"
"Oh, that's interesting. I see that there are three dots missing to fill this up"
Be yourself around your brother. You seem like a caring and thoughtful older sibling. You like mathematics. Your brother looks up to you. When you show that you care about math, he will learn to care about math.
Avoid at all costs people who like to say things like "I was never good at math", or "I'm just not a math person." And when you hear a person say such things around a young child, you absolutely must have a conversation about it with the child, the same way you would if the child witnessed a murder. Because if you don't, you might let that random comment murder the child's nascent love of math.