The answer is:
My mother is very tall but my father is even taller.
Conjunctions join clauses, words and phrases and they are usually used to avoid a sequence of short sentences. For example, and, but, and or.
In this case, the most suitable sentences to combine with a conjunction like "but" are the ones whose subjects are related (mother and father) and whose predicates have a similar structure: both describe height and one has a comparative form of the adjective tall, so they can be easily joined.