I don't know how old your daughters are but I personally don't believe in putting relaxers in children's hair. I would not subject my child if I had one to even the possibility of the kind of chemical burns I had.
If they keep bothering you to relax then they must get the vibe they can pressure you enough to change your mind. You n eed to stop engaging in these conversations with them. you let them know in a firm way the answer is no and it is not up for discussion. And every time they start shut that mess down quick. You even give the look like a mama gives when you are one move away from getting your tail beat. They will get the idea. And they will leave you alone.