You've already had loads of great advice on here op, but since my DS is almost the exact same age as yours and training right now, I'll tell you how it's worked/working for us.
The potty had been sat in the bathroom for a good few months (unused) and DH and I had been letting DS watch us as we use the loo and telling him that the potty is for him to use. Around May time he started a new habit of putting his hand down the back of his nappy while doing a poo and getting it all over his face, furniture and toys, so we suspected that he might be ready to try.
Our childminder is crap (whole other thread and excuse the pun) so from June we started sitting DS on the potty morning and night just for him to get used to the idea. He maybe had about 6 successful wees in 2 months. We were waiting for August as DH and I have the whole month off, so decided to do intensive training during this time.
We're in week 3 of the sticker chart and DS is doing amazingly. We've got rid of daytime nappies entirely unless we're out and even those are becoming increasingly dry. There have been accidents but it is definitely working. DS still isn't asking for the potty, but that will come. If he's got nothing on he'll just go and sit on it himself, but if he's going commando we can tell when he needs to go (he stops everything and looks down), so we whip his pants down for him.
I downloaded Poo Goes to Pooland and watched it a couple of times a day with him last week, and he's nailed poos ever since. It's fab and I'm sure it's helped with the wees too. DS is getting lots and lots of praise for every wee (double for a poo) and he insists on flushing the loo himself when we empty the potty.
Ignore ignore ignore your ILs. You know your child best and you should try when you think it's the right time. Don't let them pressure you or your DC as it will have the reverse effect as well as stressing you out!