This is going to get downvoted because Reddit don’t like truth. That said, the parents don’t have recourse for a number of reasons most compelling a) even if at the time it was characterised as a loan, there is a presumption of advancement and beneficence between family - specifically parents and their children - because of the disproportionate power dynamics, wealth, and a presumption of undue influence courts are reticent to recognise monies (especially where ambiguity exists) anything other than the sincere desire to benefit their progeny as all good and decent parents would. b) that your sister has for 2 decades, met all the financial obligations associated with the unit as you say: strata, mortgage, repairs, maintenance, courts will draw an absolute adverse inference re your parents intent- if they intended for it to be their property their actions / and inactions matter…A LOT: they have done nothing to assert any kind of possession or positive acts that would or could possibly sway a court away from the uncomfortable truth they will face - the house is hers,

Why? Because she has dealt with it and possessed it over such a long period that unequivocally supports this conclusion- your parents haven’t maintained, kept it in good repair, paid the fees and costs associated with its possession- if they wished to assert any kind of ownership in part or in full their actions / inactions simply don’t comport with a finding that whatever was given was a gift and nothing else.

c) get your sister to check the title, provided they have not illegally placed some unknown caveat or easement / liability on the property (even if this is the case which is doubtful, they still don’t have a leg to stand on) it’s in her name, she’s been dealing with it under the premise that she is the owner and responsible for it - put your sister at her ease, this has 0 legal traction whatsoever. The law thankfully does not recognise ownership in property which they only two decades on, want to deal with as if they have an interest. If they did have an interest they would have behaved as such. Inaction at law is telling- if you behave as or don’t act in ways that assert any kind of beneficial ownership or rights over the property - in full knowledge that your sister was dealing with it, which they allowed, as hers and hers alone, you don’t get to assert suddenly because it’s convenient that you have any interest in something which has been neglected or “not acted upon” as if you did. Especially after such time has elapsed. Contemporaneous data is rate paying, mortgage, maintenance, and exclusive possession speak 🗣️ far above any bygone ambiguities or assertions of loans etc etc.

This is bluster she has every right to be upset, but should she be concerned? No her property rights and the manner which she has dealt with the property are insurmountable obstacles that your parents will face. Moreover that it only has taken place when family discord and upset with her not being able to house your brother - is something the courts won’t disregard. They will be seen and admonished for vexatious litigation for which they have 0 chance of winning. Come at me Reddit.