Devki is not evil like Amma and has conscience, but she is no great mother. Only after Vishaka ran away to Mumbai and Radhika saved Shastriji, she changed towards Radhika.
Earlier instances of her discrimination and here I am not talking about not standing up for Radhika like during the first wedding or when she was being abused by Amma.
1. Radhika sleeping on the floor, Vishaka on the bed. She would give milk to Vishaka but not Radhika at night. Whenever she was leaving her house, she would ask Radhika to take care of Vishaka and make sure that Vishaka has her lunch or dinner or whatever.
2. She kept Radhika hidden from Dev after the first wedding so that even advertently Dev doesn't find out that he married Radhika.
3. When Radhika went to meet Dev at the ghat and told him the lie about getting married to someone else, Devki saw them together and was suspicious. She later on admonished Radhika for trying to meet with Dev and breaking Devki's trust.
4. During phool bangla, she sent Amma to Radhika and had no courage to tell her that Devki has changed her mind. When Amma slapped Radhika for trying to tell the truth, there was no reaction from Devki. Instead, she told Amma that she would make sure that Radhika understands that she can't tell the truth. Later on, when Dev asked her about what Radhika wanted to tell him, she told him that Radhika is concerned that he doesn't take care of Vishaka and should concentrate on his wife. Eventually, when Radhika wants to go back to Dev's house, she holds her hand and makes her feel guilty about trying to destroy Vishaka's life by staying close to Dev. It was another matter that Radhika at that time was shown to be strong enough to stand her ground.
5. Then the whole trying to get Dev married to Vishaka by deception and then trying to break Radhika and Dev's marriage.
6. When the PF was abusing Radhika for meeting with Purab, kept quiet and brought her back to Vrindavan.
7. Trying to get Radhika married to the old guy and then Keshav forcefully (even Devki was asking Radhika to do it).
8. Lying to Keshav when he found out that Dev and Radhika loved each other. She told him that it was just an attraction and he should try to win Radhika's love and get married to her. Actually, at that time Radhika called Devki and told her off for doing this to another person (Keshav).
9. Trying to get her married again after Radhika took off her sindoor and only backed off when Radhika tthreatened her with revealing the truth to Shastriji.
10. When Radhika left home after Dev decided to divorce Vishaka, Shastriji started going out to search for Radhika. Devki stopped him by saying that he is not well enough and not to look for Radhika. It was only after Shastriji accused her of discriminating between Radhika and Vishaka that she relented.
11. Finally, Devki only told the truth after Shastriji forced her and if he wasn't there, she would have lied again. Even then she made it seem as if she and Radhika did it to save the respect of two families whereas it was only Radhika who did that.
So, Devki in my opinion may feel Radhika's pain because she has some conscience unlike Amma and Vishaka, but she is weak and looks for her own interests. She did change later on, but there was never any recognition on her part that Dev's family will not even be associated with her family and Vishaka if it wasn't for Radhika.