Hi there,
it's totally possible to engage designers and contractor separately.
i would say the designer is the brain, and the contractors is the brawn; and they're definitely not replaceable by each other.
the designer will go thru with you their concepts, give u advice on styling and furnishings;
and provide you 2d and 3d drawings. after that, usually the designer will give u all the info, and you can proceed on to looking for a suitable contractor to do the job. (in this case you seem to have a gd choice in mind).
however, do bear in mind that in this case the designer's responsibility ends when he/she hands over the info.
and all the scheduling and oversee-ing of progress will have to be done by yourself. this can be quite tedious, esp when you're working and renovating at the same time.
some ppl can take leave and wait for the contractor to deliver the materials to their place say what time, what time. but many do not have the luxury of time and spare energy to do this, thus engaging an ID will ease some anxiety and lessen the stress of renovating.
so unless your relative is the contractor, you can trust them with your keys (if you have not moved in) or your valuables at home (minor reno at home, perhaps kitchen or bath rm reno only), I'd say it would probably require quite a bit of coordination. if not, engaging a separate contractor should be fine
hope this helps.