Point well stated, CCCharlie!
To be sure, any time you rely on public transportation, you're at the mercy of others, e.g., schedules, equipment breakdowns, etc.
Most everyone I know who lives on the Cape and flies into/out of Logan, takes the bus. It's easier than driving, cheaper than paying to leave your car at the airport while you're gone, and they know better than to ask a friend to drive them to Logan unless it's a dire emergency. ;-)
I 'm just like your in-laws. I'd rather be a super-early to the airport than risk missing a flight. So whether we're taking public transport to the airport or driving our own car, I insist on leaving at very least an hour's leeway, just in case we get stuck in traffic or something else goes haywire along the way. Goodness knows, those things do happen - even when you're in control of the driving!
The OP will be traveling from Boston to the Cape on the Saturday of Memorial Day weekend. Hopefully, by the time they get on the road to the Cape, traffic won't be an issue and they'll have smooth sailing all the way.
And the following Saturday when they're heading back to Boston, I wouldn't expect off-Cape traffic to be anywhere near as nasty as it gets on Saturday mornings during July and August.
In either event, whether they drive or take the bus, I'd advise OP to leave plenty of time to get back to Logan if they're catching a flight home on Saturday.
Although the Sagamore Bridge is undergoing repairs, all lanes on the bridge will likely be open for Memorial Day weekend travel, but not so sure about the following weekend.