what should i do

There is nothing wrong with masterbation, but using your mothers vibrator is a hygiene issue. If your mother comes in contact with any infection you could transmit it to yourself. It is also a breech of trust to use her personal things without her knowledge. I am sure you wouldn't like it if someone else was using your toothbrush. You might want to talk to her about getting your own vibrator, or buy one yourself.

In all honesty i'm pretty sure she will know already not least of all because the batteries are allways flat, with my daughters it was pretty obvious for, me i waited till they brought it up and then bought them their own, if she uses one then your mum will probably be pretty cool about it