If they stick good. The first few generations are almost sure to mix hindu and sikh ways.
Some people just spend their whole lives living on the fuzzy borders between sanatan dharam and sikhi . Others slowly adopt. I know haryana people who first stepped into sikhi by starting to go to gurudwara . Some faltered away back into hinduism, others sticked and now their kids are kesadhari.
I also know of some destitute women with many kids, who stayed in gurudwara , helping in langar and sewa and slowly adopted sikhi .
Our hindu ancestors most likely stepped into sikhi thinking of it as a sect of vedantic hinduism. They certainly didn't see it as some alien religion .