When looking at it only from the technical/mathematical side, the answer should be fairly easy:

- Compile a list of all the debts, ranked from highest interest to lowest

- Payoff all debt above and equal to 6.39% interest

The rationale is quite simple: Why paying for money at a rate of x% if you have money available that earns less interest (assuming that the 6.39% is a fixed interest rate and not an average return rate).

However, as Cossack has stated already, advice among relatives is always a bit difficult. Maybe pure math helps. Sometimes also framing it as questions (e.g. "What is your investment rationale for xyz...", "What are your (financial) plans for your retirement together") helps as well.

For now, I would focus on one issue at the time (e.g. interest rates). Try not to mix the discussion with criticism on their lifestyle and expenses and be open for ideas. To be honest, even consolidating debt at an interest rate <6.39% would make economic sense in this fixed income situation.

If that subject is tackled, you can move on and introduce one of the other topics (e.g. spending) gently after some time.