Contact details in Mortsel are here. You should be able to get Anna's birth certificate since the event was more than 100 years ago. I would contact the email address listed on that page and explain the situation.
http://verdonk.home.xs4all.nl/aw.htmhttp://verdonk.home.xs4all.nl/aw.htm - someone here seems to have some information on Anna's parents, listing them as: Ludovicus Jacobus Eduardus Verdonck and Josephina Francisca Ooms, with quite a bit going back on the Verdonck side. (it has the marriage to Frederick and the sister Rosa listed)