Difference between koi and ai, aisuru


I'm trying to pin down the connotations of the word '愛する', romanised as
'aisuru'.

I had read that there were two forms of love - koi and ai - in a manga once.
The original Japanese went - 'if you're alone, koi doesn't become ai!' And the
English equivalent was 'true love is for two people!'

So my question is, is ai for romantic situations only or not? Is it deeper
than koi, which I had believed meant 'infatuation' or 'crush'?

What would be the connotation using 'aisuru' when referring to the love
someone has for someone else?

E.g. If I said to someone 'your love for Jane/John' using the word 'aisuru'?
Could that someone love Jane/John as family or platonically using 'aisuru'?

If you have any points related to Japanese culture, please feel free to
include them in your answer!

[Edit: different than proposed in that a third party is the one speaking about
the love between two other people, and I am concerned with 'aisuru'
specifically].

