YSK the only person who can break your boundary... is you!
Why YSK: People misuse the word all the time, but technically...
A boundary is an IF/THEN statement, with the THEN being something that you yourself have control over.
So if someone does the IF, the way to hold the boundary is to then do the THEN. Only if *you* fail to follow through on the THEN is the boundary broken.
For example, I may say to my toddler, "If you throw sand in my eyes again, I am going to walk away and not play with you anymore."
That is a boundary. Him throwing sand does not make or break the boundary though. Me walking away or failing to walk away does. He is welcome to make his own choices but he can't choose the consequences. A boundary with unenforced consequences isn't a boundary at all though. It's just a rule that can and will be ignored.
Another example: "Sister, if you don't give me at least a week advanced notice, then I will not make time to babysit for you unless it is a real medical emergency."
In this scenario, if the sister then calls you last minute to babysit for her to have a girl's night out... that is not her breaking your boundary. It's not her job to hold the boundary; it's yours. So you then maintain your boundary by saying, "Sorry, like I warned you, I need at least one week advanced notice. So I will not be babysitting for you tonight." (Regardless of whether you are available to babysit.) That is what having a strong boundary looks like. She can't break that boundary, because you don't let her get away with trying to push that boundary because you follow through on your THEN.