I think I had the wrong guy with that World War I draft card.
The guy you found in the 1920 census in Washington DC is Frank Lifka.
He shows up in the 1930 census in Northampton, Mass with wife Mildred and son Reginald. His World War II draft card gives name as Frank Andrew Lifka, birthdate as 22 Jun 1890, Lapant, Czech-Germany; says wife is Mildred, working at College Candy Kitchen.
Same guy, Franz Lifka, world war I draft card, born 22 Jun 1890 in Labant, Bohemia, single, living in Boston, working at Erhardts Schatz in Boston.
14 Sep 1922, Franz Lifka coming to Washington, DC to see brother Andreas Lifka. Leaves father Ferdinand Lifka in Muenchsfeld, Czechoslovakia. Gives birthplace as Labant Muenschsfeld. He had been in Washington before, from 1914-1922.
Whether this Franz Lifka is the Franz Lizka that you're looking for, I don't know.
You can either look for Franz Lizka-s in Washington DC or look for Franz Lizka-s emigrating from same place as your grandfather or look for Franz Lizka's marriage record in your grandfather's birthplace or last residence. A brother-in-law can either be your wife's brother or your sister's husband (if you're a guy). If your grandfather was single when he emigrated, then Franz must be his sister's husband. If he was married, then you can't tell.
But given his age in 1920 Census, that guy, whoever he is, should be in world War I draft, and World War II draft if still living.
Your grandfather's obit should list surviving siblings, so should your grandmother's obit.