I have been researching on the subject recently.
Never ever people of Beresteyschina identified themselves as Ukrainians or have been part of the Ukrainian ethnicity or the Ukrainian state. People of Beresteyschina are not ethnographically more similar to Ukrainians than to Belarusians.
People of western Polesia speak a transitional dialect between the Belarusian and the Ukrainian languages. Some linguistic features of their dialect are similar to Ukrainian, while other linguistic features are similar to the Belarusian language. Some linguists stated that their dialect can be considered as a dialect of Belarusian or Ukrainian, while other linguists suggested the dialect of western Polesia can be considered as a micro-language.
A brief overview of history of Beresteyschina
*When I will state Ukraine, Russia or Belarus in historic context, I will mean the territories of present day Ukraine, Russia or Belarus.
The region of Brest was settled by eastern Slavic tribe Dryhavichy and western Baltic tribes Yotvingians for the most part. The city of Brest was founded by the eastern Slavic tribe Dryhavichy as per several leading historians. Sources can be provided if needed. Dryhavichy was one of the founding Slavic tribes of the Belarusian ethnicity.
The city of Brest and the region Beresteyschina became part of the Turau-Pinsk principality incorporating the territories of southern, western and central Belarus. The other stronger principality on the territory of Belarus was the Polatsk principality competing against Novgorod and Kiev at some point in time.
For a short period of time, the Volhynian principality became strong and dominant in the region fighting several wars with the Balts (Yotvingians & Lithuanians) on the territories of present day Belarus. During that time Beresteyschina was part of Volhynian principality. But only for a short period of time.
The Mongols invaded (1238-1240) principalities of Rus on territories of modern Russia and Ukraine creating a political vacuum on territories of present day Belarus. Belarus was free of Mongol invasion. Princes of principalities on territories of Belarus were afraid of Mongol invasion reaching political agreements and entering dynasty marriages with the Lithuanian princes, who had a powerful army in the region at the time. The Lithuanians were familiar neighbours, who were culturally similar despite being pagan.
Most of the territories of present day Belarus were incorporated into the Grand Duchy of Lithuania by 1320, while several regions of western Ukraine were already part of the Polish Kingdom. The city of Brest was among the last cities to join the GDL peacefully in 1319.
During Lublin negotiations (1569) and Polish-Lithuanian Commonwealth formation, most territories of present day Ukraine were passed over to Poland as part of the agreement, while all territories of present day Belarus including the region of Brest remained in the Grand Duchy of Lithuania. Nobility on the territories of present day Belarus were loyal to Lithuania, while Ukrainian nobilities sought privileges of Polish nobility.
The next import period of history was the partition of the Polish-Lithuanian commonwealth and incorporation of the territories into the Russian empire. In Russian empire, the administrative division was such that Beresteyschina and Brest city (Brest-Litovsk back then) was part of the Hrodna governate (Grodnenskaya Gubernia). The Hrodna governate included all territories of western Belarus and few territories of in north-eastern Poland around Bialystok region. After disintegration of the Russian empire and formation new Poland of which western Ukraine and western Belarus became part during inter-war period, there were Polesian and Volynian Voevodships. The Polesian Voivodship was entirely on the territory of present day south-western Belarus which included the Brest region (Brest, Pinsk, Kobryn, Slonim and other).
After Poland was partitioned between USSR and Germany, Polesian Voevodship and Brest-Litovsk joined the Belarusian Republic (BSSR). There was a vain attempt by Nikita Khrushev – not the brightest politician – to hand Beresteyschina over to Ukraine in 1939. He succeeded in handing over Crimea to Ukraine in 1956. The Belarusian delegation collected all evidence presenting it to the Soviet government in Moscow showing that Beresteyschina was historically an integral part of other territories of Belarus. So the region remained in the Belarusian republic (BSSR). After the disintegration of USSR, Beresteyschina remained part of Belarus. The modern day border between Belarus and Ukraine is approximately what it was between the Grand Duchy of Lithuania (Belarus) and Polish Kingdom (Ukraine) during Polish-Lithuanian Commonwealth.
As you can see, people of Beresteyschina lived in the same borders with people of other territories of Belarus for around 1,000 years. Since Dryhavichy founded the city of Brest. Ethnographically western Poleshuks are more similar to the Belarusians than to Ukrainians. Belarusians are a homogeneous society which cannot be said about the Ukrainians. People of Brest are similar to people of Homiel region (south-eastern Belarus) or Viciebsk region (northern Belarus) than let’s say to people of Podillia (south-western Ukraine). The language of western Poleshuks can be considered as a micro-language. More importantly, modern day people of Brest region identify themselves as Belarusians.
Demography of Brest region as per national census 2009
Belarusians – 88%
Russians – 6.4%
Ukrainians – 2.9%
Poles – 1.3%