First and foremost does your mother have capacity to make a POA?? If she does then it is her decision as to who she chooses as attorney but she can only choose people who are willing. If you refuse to be an attorney jointly with your brother because you cannot work with him cooperatively she cannot insist.

To have capacity to make a POA she must be able to:
1. Understand information given to her in relation to the specific decision. That would include her diagnosis, the fact that dementia is incurable and gets worse, the probability that she will soon or already be unable to manage her own finances. I think it also includes the fact that you and brother cannot work together and that he is often abroad, which would make it more difficult for him to perform the role. Having the two of you managing in harmony is not an option so she has to understand that.
2. Remember the above for long enough to make a decision
3. Weigh up the pros and cons of all her options, which are basically choose your brother, choose you, or choose someone else or choose a professional like a solicitor. She also has the option of having a solicitor as a backup in case the main attorney is incapacitated.
4. Communicate her decision

If she can do all that, with help, she decides. If she cannot do one of those four things she does not have capacity to make a POA and you need to apply for deputyship.

You do not need someone else to tell you that she has or has not got capacity for the decision. You can judge for yourself. However, because of the possibility of future conflict here there would be an advantage in having a solicitor in draw up the POA and going through steps 1-4 above if only so that you cannot be later accused of undue pressure on your mum.

If she does not have capacity, apply for deputyship for you alone - another subject!

If she has capacity and if you were to be a joint attorney with your brother the chaos could be that you might arrange a care plan on a Monday and he could cancel it on Tuesday. You might decide that the time had come for a care home and he might take her back home. He might decide to have her kitchen refitted and you might cancel it as not wise or necessary at her time of life. And so on. The scope for conflict is endless.

If you think your mum has capacity to make the POA I suggest seeing her and alone and explaining that she has to choose you or your brother. Not both. That you refuse to work with your brother. That's going to be hard for her but it is unavoidable.