python extracts n sentences from a string after the first instance of the string has been found

For extracting the first instance, your code does work but has some unnecessary code.

Simply append i, not i.split('.')[0]. Your i is already splitted.

for j in words:
    for i in text.split('.'):
        if j in i:
            lst.append(i) 
            break

Now for extracting n sentences,

for j in words:
    for index, i in enumerate(text.split('.')):
        if j in i:
            if j == "dog" or j == "cat":
               lst.append('.'.join(text.split('.')[index:index+n]))
            else:
               lst.append(i) 
            break

Here using enumerate, we track the index of the sentences. If we encounter a sentence with dog or cat, instead of appending we add the sublist with length n.