May be because now her world is shattered just with 1 ruth out her faith in her husband, her love for Yash, her position in the family has changed. What is she supposed to do now? Just because Yash is Radha's son, can she push him out of her heart and replace him with Akash, her biologicl son? She had showered her love on Yash for the last 30 years and can't change it now. If she accepts Akash hs her son, yahs will feel he is left out, if she doesn't accept his, Akash will be left with illegitimate tag despite not being one. If she accept both Yash & Akash as her son's where will radha go? Will she be able to ask her to leave on her own by leaving her child with Gayathri? NO, being a son, as a biological son, Yash will opt to go with Radha as he can't think of his mom ( even if he can't accept on the face value, as a responsible human, he can't expect a lady deprived of a sons love), at the same time Akash knows what all the hardship Radha went through to bring him up and he will never be ready to grab his luck by leaving a shattered mother at bay. So even he will say either they accept him along with his mom or allow them to leave to live with their fate.
DO you expect Gayathri to open heartedly invite Radha and share her husband (even if she hates SP now) with the other.
All these dielemma is because of Aarti's outburst of truth which has shatteredher entire world.
Jaya