documentclass[a4paper]{article}
usepackage[utf8]{inputenc}
usepackage{libertine}
renewcommand*familydefault{sfdefault}
usepackage[T1]{fontenc}
usepackage{linguex}


begin{document}

section{Sans serif font: Libertine Biolinum}

Text text text text text

ex. Linguex numbered example without interlinear glossing
a. Every layer
a. appears in
a. sans serif font without interlinear glossing

But as soon as I use interlinear glossing

exg. Both object language and \
glossing appear with serifs\
'Last line is sans serif again.\




end{document}