Currently on my team, I got
Why do I get knocked out with one hit when ever I battle my sister? And why do her Pokemon avoid my attacks all the time? I am very mad!
I have a few theories to why you lost. Also, just because you lost doesn't mean your team is weak.
Your sister got lucky and earned better EVs and got better IVs. (I assume you don't EV train?)
You got haxed. This mean you were critted a lot, got hit by chanced status, your attacks missed, ect.
Your sister's Pokemon are higher levels and fully evolved.
You have a massive water and fighting weakness, so maybe she used a bunch of super effective moves.
Well, those are a few things that could have caused it.