Income from rental property tax

[clearsky]

My wife is a part owner of a property which was transferred to her by her father in 2012 (approx. value £120k). She has a 50/50 ownership with her brother.
The property has been let since June 2012 and has a monthly rental income of £575 of which she receives 50% (£287.50). There are a few annual maintenance costs associated with the property so once deducted the income is approx. £3250 per annum.
Her brother is self employed and hence his half (which he has since remortaged) is tied up in his self assessment declaration.

We had been told that as the rental income fell below the threshold limits that she did not have to complete a self assessment. We were also advised that this is due to the fact that this income can be shared between my wife and myself, thus keeping below the £2500 limit.

We are both employed, my wife part time with an income of £21,020 and tax deducted £1,902 (from 05/04/18 P60). I'm employed full time income of £40,875 and tax deducted £7,475 (05/04/18 P60) (I have a company car benefit (£6,104 from P11D).

My wife has a 1/3rd ownership on another family property which she is looking to start renting out which will generate an income of approx. £300/ month

Would welcome advice on whether we are acting correctly in our existing situation or having now looked in more detail we need to be declaring the income by self assessment? Are we correct in thinking this income can be shared? What will be the implication of the potential new additional rental income?

Any advice gratefully received as somewhat confused
Many thanks in advance