A frequent question is "can my father/brother/cousin" get SEIS or EIS for their investment? (where that's the father, etc., of one of the founders).
The short answer is that
spouses and civil partners,
parents and grandparents,
children and grandchildren
of anyone with a controlling interest in the company (30% or more of the shares or voting, or an employee of the company) cannot get SEIS or EIS.
Note that separated spouses or civil partners are regarded as associated with each other (so that if one partner is associated with the company and is ineligible for SEIS/EIS, then the partner will be ineligible too) but divorced persons are fine. Mire info on the HMRC site here and, in relation to transferring shares between partners, here.
So, when doing your SEIS or EIS Advance Assurance, don't be tempted to put a father or mother as an investor for the purposes of having at least one SEIS investor for your Advance Assurance application, it'll get rejected if HMRC spot the connection.
On the other hand, brothers, sisters, uncles and aunts are all okay.
Also, anyone who's an employee of the company will not qualify for SEIS or EIS - there's an exception if the person is an investor who becomes a director of the company as part of their investment.
For a full breakdown of SEIS/EIS rules for investors, read: SEIS & EIS Rules for Investors: The Complete Guide