Hmm, not sure if it's "normal", I think every sibling relationship has it's own dynamic.
I agree with this.

DD is 4 (5 in May) and DS is 2 (3 in July) so just over two years - they get on better than I expected, she is incredibly tolerant with him and he loves playing with her. She is very motherly towards him, but also loves the fact she has someone to take part in her imaginary play. When he doesn't do what she wants she can get a bit fraught initially but generally works out how they can get round it. She doesn't play with that much that he would mess up though - she is more into lets pretend we are pirates etc, and if she is doing Hama beads or something - often she will go into 'teacher' mode with him. He does wreck things sometimes, but she just seems to roll her eyes and not get too frustrated (she is more likely to get frustrated if he doesn't comply in some scenario or other) than if he wrecks something. I do think some of the tolerance is from childcare, when it is outdoor play (which if not raining is prob 50% of the day) there is a mixture of ages in the outdoor space with a range of up to about 2.5 years - so they have always been used to having older, and younger children around - now DD and DS are in the same outdoor area and I do get some 'tales' back from DD about DS - but they still play together sometimes even there. They even don't like it if one of them gets told off, they stick up for each other and it feels like they are 'ganging' up on me.
My brother and sister had the same age gap as they do though, and never got on particularly well.

I really do think it is a personality thing how well they really get on - how is your DS1 with sharing his things and playing with other friends - is it only his brother he is like that with? What about out of the home e.g. at the park or something where there are no 'things' are they the same there?