The TV judge is married to Ukrainian dancer Evgenij Voznyuk and his parents are still in the country, which has been invaded by Russia.
Mabuse, 40, said her relatives have "no possibility to get to the Polish border" and that it was "heartbreaking" hearing her mother in law cry.
The star — who divides her time between the UK and Germany — tweeted: "It’s heartbreaking hearing my mother in law crying this morning, afraid and there absolutely nothing we can do!
"Just sad honestly."
She later added: "Our parents have been sold (sic) to go outside and to switch off water & gas!
"They have no possibility to get to the Polish border."
The dancer previously revealed she had urged her parents in law to leave as tensions escalated.
She was quoted as saying earlier this week: "Me telling my parents in law to come to Germany from Ukraine. They saying they are not worried!! Really confused right now."
Fans flocked to express their support for Mabuse after she shared her fears about her relatives.
One tweeted: "So sad! Our thoughts are with your family and all the Ukrainian people."
"Aah, Motsi, so many of us thinking and praying for you and your husband and his family at this time," said another.
One fan wrote: "How devastating is that. I’m so sorry.
"I hope they are able to get to a safe place."
Mabuse and Voznyuk were first romantically linked in 2015 and they tied the knot in 2017.
The pair, who have a dance school in Germany, welcomed their daughter in 2018.
Mabuse spends part of her time in the UK, where she is famous for Strictly, and part of her time in Germany, where she is known for the programme Let's Dance.
Watch: Strictly's Craig Revel Horwood on Oti Mabuse's exit