In my native language we have this expression for a weak person - " a low fence " implying that anybody can jump over it. Is there a similar expression or idiom in meaning in English for this type of person ?
We might call a person like that a pushover, which means someone who is easily overcome. You might use it like
You have to learn to stand up to your brother! Don't be such a pushover!
In the U.S., we sometimes use "doormat" for this situation, which fits in meaning with another idiom: weak people "let others walk all over them."
Here's a snip from the COCA, talking about disgraced politician Anthony Weiner:
"The doc, destined for theaters and living rooms on Showtime, is about the rise (sorry again) and crashing and burning of the former congressman who threw away his House seat, his dignity, his employability, but somehow kept his doormat wife, for the sleazy thrill of sexting pictures..."
It's being used here as an adjective, but there are several other examples there where people and sports teams are referred to as "doormats" (noun).