Your sentence is an example of a runon or comma splice, which is probably why you feel that there is something wrong with it. In English classes in schools, students learn how to recognise and avoid such problematic syntax.
Basically a comma splice is two independent clauses, each with its own subject and predicate, joined by a comma. A simple example is:
?I love Paris, it's a beautiful city.
In your case, the two independent clauses are:
With my brother standing by my side, I reached for the pot handle
Tilting the pot way too much caused the boiling water to spill
(subject: Tilting the pot way too much)
The way to fix a comma splice is to replace the comma with a full-stop or semi-colon. Alternatively, you can sometimes insert a conjunction. For example, but might just about fit here:
With my brother standing by my side, I reached for the pot handle, but
tilting the pot way too much caused the boiling water to spill.
Note that comma splices are not always considered wrong. There is a good short article on the topic on ThoughtCo: https://www.thoughtco.com/what-is-comma-splice-1689897