Hi Miraclecraz,
For my case, either (1): don't tell OR (2) when you tell, be prepared to listen to your mum even if you disagree. I'm Christian and definitely don't believe in all this superstitions but out of respect to my mum and to let her have peace of mind and maintain the happiness at home, listen to her lor.
I have personally encounter a lot of such stuff recently.
Eg, my boss's FIL passed away last week (just as I was about to distribute invites to my colleagues). Told my mum and I was banned from giving him invites and also attending the wake. (Explained to my boss, luckily he's understanding about these things). Haiz, wasted 1 card there though.
My flowergal & pageboy's grandfather also passed away recently but I didn't tell my mum that so am continuing with them.
Actually, have you spoke to your jiemei yet? It may happen that she wish to opt out coz she needs some time to recover from her grief? Then in this case, all the disagreement between you and ur mum may be for nothing. Just a thought.