Yes it was common. But given the popular reason that the war was supposedly to be fought, it becomes hypocritical of the Union officer corps and of the later "historically" avowed purpose of the northern war effort.

And by the way U.S. Grants father-in-law did own slaves as did his daughter when he gave them to her, which under the laws of marriage of the day became the property of her husband. Who I believe was discussed on this board some time back that Grant sold at least one of them in Missouri. Or maybe it was Grant's own slave that was sold and not his wife's. In any case the trail of the Grant family ownership of slaves become quite vague in 1863 for some reason as to whether they were released prior to December 1865, and the 13th amendment, or not.

The sister of Mary Todd, wife of Abraham Lincoln, married Benjamin Hardin Helms who became a Confederate Brig.-Gen killed at the Battle of Chickamauga. Benjamin Helms who was considered as Lincoln's favorite brother-in-law, was offered a commission in the Union Army by Lincoln, But refused the commisssion to join the Confederate cause.