Hello friend,
Don't mind your family. Most meditations (especially most of those starting out) are completely silent. The only thing others will see is you breathing deeply, if that. As for the mantra meditations, those will take 5-7 minutes at most per day in your 40-day program. Surely you can turn on the shower and use its sound to conceal your mantra vibrations, or get some privacy to do it in that short time.
Outside or inside doesn't matter, so long as you can relax and focus. Secondly, meditation does not belong to one single religion alone. Well, anything spiritual does belong to us Zevists at the end of the day, but almost nobody knows that and they don't have to either. You can simply tell your parents about the myriad benefits of meditation (there are lots of scientific papers on it, and HPS Shannon has recently made a topic including scientific info:
https://ancient-forums.com/viewtopic.php?f=5&t=18620 ), and broach it to them purely from a secular angle. Again, anybody can do meditations. One does not have to be a Zevist.
If your parents aren't the understanding type, then just don't bother and do it where they don't see you. When you get strong enough, at worst, you can do a harmless spell on them to make them more understanding about these topics, maybe.
Hope this helps.