Translation:My sister travels a lot, so she saw a lot.
The sequence of tenses should be observed in English. Therefore the matched phrasing should be for example: "My sister travels a lot, so she has seen a lot." or "My sister has travelled a lot, so she has seen a lot." or "My sister travelled a lot, so she saw a lot."
This Ukrainian-from-English programme needs to be reviewed by a native English speaker.
"My sister travels a lot, and so she has seen a lot." should be accepted. Do not try to keep the two separate clauses in the same tense. In English it makes perfect sense to have the first clause in the present and the second in the past. Because Ukrainian does not have the wide variety of past tenses that English has, both the accepted translation and the one I have given are equivalent.
I have three Ukrainian dictionaries that all give "and so" as the first translation of "отже". To the people who think that one cannot use past and present in English, you are wrong. The two verbs are in separate clauses. There is no requirement in English to have tenses of verbs in separate clauses agree.