Correct grammar for "While speaking Cantontese with my mom, I accidentally said one half of a sentence in Mandarin and the other half in Cantonese."?
Mandarin has sure enough affected my Cantonese. &quot;While speaking Cantontese with my mom, I accidentally said one half of a sentence in Mandarin and the other half in Cantonese?
我國語果然有影響我的廣東話。今天我跟我媽媽講廣東話，我無意講的一個句子有一半國語也另外一半有廣東話。

我的国语果然会影响我的广东话。今天我跟我妈妈讲广东话时，我无意讲的一个句子，有一半是国语，另一半是广东话。
The way to express such a complex sentence is, split the phrases out.  For example

I accidentally said one half of a sentence in Mandarin and the other half in Cantonese

is
“I said a sentence.” “The sentence is half Mandarin and half Cantonese.”
我说了一个句子，句子里一半是国语另一半是广东话。
Though yours is good enough to understand, personally I would suggest following translation:
我的粤语（表达）的确被国语所影响。跟我妈说粤语时，我无意间说出了国语粤语参半的一句话。