This is controlled by directive 2004/38/EC:
- This Directive shall apply to all Union citizens who move to or reside in a Member State other than that of which they are a national, and to their family members as defined in point 2 of Article 2 who accompany or join them.
- Without prejudice to any right to free movement and residence the persons concerned may have in their own right, the host Member State shall, in accordance with its national legislation, facilitate entry and residence for the following persons:
(a) any other family members, irrespective of their nationality, not falling under the definition in point 2 of Article 2 who, in the country from which they have come, are dependants or members of the household of the Union citizen having the primary right of residence, or where serious health grounds strictly require the personal care of the family member by the Union citizen;
(b) the partner with whom the Union citizen has a durable relationship, duly attested.
The host Member State shall undertake an extensive examination of the personal circumstances and shall justify any denial of entry or residence to these people.
Here's what article 2, point 2, has to say:
- ‘family member’ means:
[Items (a) through (c) are not relevant.]
(d) the dependent direct relatives in the ascending line and those of the spouse or partner as defined in point (b);
The question of dependence is mentioned in the prefatory material:
In order to maintain the unity of the family in a broader sense and without prejudice to the prohibition of discrimination on grounds of nationality, the situation of those persons who are not included in the definition of family members under this Directive, and who therefore do not enjoy an automatic right of entry and residence in the host Member State, should be examined by the host Member State on the basis of its own national legislation, in order to decide whether entry and residence could be granted to such persons, taking into consideration their relationship with the Union citizen or any other circumstances, such as their financial or physical dependence on the Union citizen.
So, if they are financially or physically dependent, they fall under article 3(1); if they are not, but are members of the household, they fall under article 3(2)(a).
The criteria for determining dependence and household membership are left to the individual countries. There may be court decisions constraining this, but I checked the "affected by case" list on the document information page, and didn't see any relevant cases.
As an example, the UK disucsses this in EUN2.9:
EUN2.9 How do I establish dependency under the EEA Regulations?
Direct family members must be wholly or mainly financially dependent on the EEA principal to meet his or her essential needs in order to qualify for an EEA family permit, (children under 21, spouses, civil partners do not need to provide any evidence to show dependency on the EEA national).
Extended family members must be wholly or mainly financially dependent on the EEA principal to meet his or her essential needs in order to qualify for an EEA family permit (durable partners do not need to provide evidence to show dependency on the EEA national). Emotional dependence to the EEA national would also be expected in order for an extended family member to qualify for an EEA family permit.
Whilst the following criteria are not in themselves grounds for refusal, they should be taken into consideration when assessing dependent relatives:
- Whether there are any other close relatives in the country of origin from whom the family member receives material support. If a family member receives funds from the EEA national but, for example, is living in the same household as another relative who provides their food and accommodation, the family member cannot be said to need the financial support of the EEA national in order to meet his / her essential needs.
- Whether the family member leading an independent life. For example, if a direct descendent 21 or over is married (and especially if they have children), it may be questionable as to whether the EEA national is supporting the essential needs of both the family member and their spouse and children. In such cases additional attention should be paid to ensure that the financial essential needs of the family are being met by the EEA national.