There are a number of issues here not just from an OPG point of view but their guidance can often be at odds with LA regulations which can be another kettle of fish. I know you say your mum from a funding point of view will always be self funding but the next person reading this thread might take the rules as applying equally to them, when they will not.
With regards to your son, is there evidence that such payments were made regularly by your mum prior to you becoming LPA. Check the bank statements to see. If your sibling was to report you and someone investigated then it wouldn't be sufficient for you to argue that this was paid in cash and only known about verbally. In the grander scheme of things £60 per month wouldn't appear to matter in this case but it highlights the dangers of verbal agreements that have no supporting evidence. Just because a mother says that you are precious and should live in a castle doesn't mean you can go buy yourself one when you take control of finances! So evidence and intent are often called for. Does your sibling have children? Is there a reason runny wasn't so generous to their kids? The best way to protect yourself as you would be surprised how many people get reported on for trivialities, is to send an annual statement of intent to the OPG detailing the gifts you intend to distribute during the following year on your mum's behalf. Include one off gifts such as a wedding gift or special birthday gift as well as regular gifts. The OPG can then agree this and give you the authority/protection you want.
Just to close the circle for those that might be involved with a LA now or in the future, whilst the OPG might give consent within their rules for gifts to be made, the LA might take a rather dim view if such expenditure breaches their rules, especially if it turns out that LA funding for care becomes necessary at some point in the future. Don't forget that in some cases people with dementia can be in a care home for some 10 or 15 years. Today's self funder can indeed become tomorrow's LA funded placement. Despite what people think, there is no time limit that the LA is restricted to in retrospectively checking someone's finances. The 7 years many quote does not apply to the LA who, now that money is short, are investigating and pursuing people for smaller and smaller amounts of money.
As to invisibles, only they know what their motivations are for staying away but some DO have genuine reasons for doing so. It is not always the case that they don't care. With regards to what is often interpreted as greed or self serving financial interests, you just need to look on here to see that they might be justified in their concerns either for their relative's best interests or their own inheritance. This forum alone is awash with tales of theft and misappropriation of funds that would make your hair curl. So perhaps they feel justified in taking a healthy interest. Before everyone jumps on me, I am not saying there aren't criticisms that aren't justified but the bad press that some get on here is only ever one side of what might be a harrowing story and the other side is never heard on here. Yes, be careful of their motivations and protect yourselves in matters to do with money. That, in my view, is about as much energy as you should spend and leave them to their own reasoning/justifications.
Fiona