Let's start with the definition of a language family: A language family is a group of languages that descend from a common ancestor.
In historical linguistics, the best way to demonstrate that a group of languages belong to the same family is through something called the comparative method. When languages are related, there are systematic correspondences between them. By examining these correspondences, we can make hypotheses about what the common ancestor looked like and how its descendants changed over time to take the form that we are familiar with.
Used correctly, the comparative method will fail if the languages aren't related and there aren't any systematic correspondences. However, there is always a gray area. Systematic correspondences are lost over time. There are some languages whose relationships we're very sure of, some languages whose relationships are absolutely mysterious, and a lot of languages in between - there might be some evidence, but it's debatable or open to interpretation.
This is all general information but is important to know for understanding why Dacian and Thracian pose some problems. So, on to your specific question.
Dacian and Thracian are both extinct. We also don't have much surviving text from either of them. This means that we don't ave much data to compare to other Indo-European languages to see where in the Indo-European family tree they fit. There are actually several competing proposals. Several linguists have proposed that they belong to the South Baltic branch of the Indo-European family, and some have proposed that they belong to their own branch. It's not even clear how closely related they were to each other! There is no widely accepted consensus here.
Those that have proposed they belong to Baltic do so based on systematic correspondences between sounds in surviving Dacian and Thracian words and Baltic languages like Lithuanian. However, there is not enough surviving evidence from Dacian and Thracian to demonstrate that the correspondences are actually systematic and that both are descended from a Proto-Baltic ancestor language.
So the answer to your question is slightly different than the one you were looking for. As far as we know, Dacian and Thracian could be Baltic, but they might not be.
(It's worth noting that there are a lot of open questions about how other Balto-Slavic languages should be grouped, so there are all types of uncertainty in this question.)
