Xiaobaitu - question about greencard application for parents

I am planning to apply for the green cards for my parents after I become US citizen.

My dad will come to US this year and my mom could not. If I apply for green card for my dad once he comes over, will my mom have problem getting B1/B2 visa in future?

My delimna is that they could not come to US together because one has to stay home to take care of my grandpa. Any suggestion?

Thanks a lot.