Whole brain radiotherapy / second opinion

First time poster so I hope this is in the right place.

My lovely mum has been diagnosed with secondary breast cancer having had surgery to remove a lump two years ago and being given the all clear. Unfortunately, she had a strange turn at the end of June and after weeks of biopsies which were inconclusive it was discovered that the cancer had returned and spread to her brain and lungs. She also developed blood clots on her lungs which meant two weeks in hospital being very unwell. At this time she was given a prognosis of 3 months, slightly more if she got better and could manage whole brain radiotherapy. After being cared for at home she is now much better  and her oncologist has put her forward for whole brain radiotherapy - and she will be fitted for a mask next week. She is also having some hormone treatment injections starting next week. I’m obviously very keen for her to be with us as long as possible, but I’ve heard some worrying things about the side effects of the radiotherapy and wonder if it is worth this for her. We have had limited time with the oncologist and I’m considering a second opinion so we are better informed - perhaps privately. It’s so stressful as a family trying know what best to do without feeling we are really in the picture, and time is against us.

Any advice really appreciated thank you.