You should be able to set up an online account without too much information. Then just start paying it. If the in laws pay it once in awhile too, then great, it gets paid off faster.
If it's truly not doable for you right now, set up the account and monitor whether payments are being made. If timely payments aren't made, then you make a payment.
Will they find out? Yes. Or, if you want, you can probably just change the address online too.
Have a conversation or don't. If you really can't afford the loan, then clearly you need to have a conversation of some sort. Tell them that you want to start paying the loan, but need them to contribute X amount (whatever your shortfall would be). Then ask them to contribute it to you directly, since it will be simpler for you to just make one payment.
Personally, I'd do what I could to just straight up make the payments without their involvement - because you can't really count on them.
If you do have a conversation, that conversation doesn't have to involve any discussion of late payments or flakiness. Just, "We're going to start auto-debiting this. We appreciate any additional contributions you want to make, because it will be a big help in getting the balance down, but you're not under any obligation."