I they probably did end up one big happy family. However, I think Diana was referring to Sean as "Dad", not Nick. The reason you mentioned made sense, but there's also something else that seems to point toward Sean rather than Nick.
It's not unreasonable to believe Nick is still the wesen hunting grimm after 20 years. After all, there was nothing stating he retired and his mother appeared to be in her 50s and was still going strong as a grimm. '
So it wouldn't make sense that Diana would would refer to Nick as (Dad) and proceed to tell Kelly that Nick (Dad) is coming along with them to kill wesen. Obviously Nick would be coming with them routinely, probably whenever they went out hunting.
It's more likely that Sean and Adalind do not routinely attend wesen killing sprees and if they were coming along, Diana would tell Kelly, "Mom and Dad" are coming. It's all a matter of opinion, but a spin off series could include more than just Diana and Kelly and the triplets.
It's not unreasonable to believe Nick is still the wesen hunting grimm after 20 years. After all, there was nothing stating he retired and his mother appeared to be in her 50s and was still going strong as a grimm. '
So it wouldn't make sense that Diana would would refer to Nick as (Dad) and proceed to tell Kelly that Nick (Dad) is coming along with them to kill wesen. Obviously Nick would be coming with them routinely, probably whenever they went out hunting.
It's more likely that Sean and Adalind do not routinely attend wesen killing sprees and if they were coming along, Diana would tell Kelly, "Mom and Dad" are coming. It's all a matter of opinion, but a spin off series could include more than just Diana and Kelly and the triplets.
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