In a dvanda samāsa of words ending in the short ṛ and expressing relationship through study or blood, the ānaṅ is the substitute of ṛ of the first member.,
Thus होतापोतारौ, नेष्टोद्गातारौ, प्रशास्ताप्रतिहर्त्तारौ मातापितरौ, याताननान्दरौ ॥ The substitute is really आ, the न् is useful only to prevent the application of 1.1.51, by which a र comes after every simple vowel substitute of ऋ ॥
Why do we say 'both words ending in ऋ'? Observe पितृपितामहौ ॥ The anuvritti of पुत्र is understood here, so that the ऋ of the first member is changed to आ before पुत्र also, though it does not end in ऋ. As पितापुत्रौ, मातापुत्रौ ॥,