Unfortunately (actually… let's say fortunately) homemade yoghurt still contains a lot of active cultures which will continue to work even after you refrigerate it. Refrigeration will slow that process down, but it will not stop it completely.
There are two (well, three) things you can do to avoid that tart flavor from the active cultures at work:
Refrigerate a portion of the batch early
When you (or rather, your mum) is making a batch of yoghurt, refrigerate a portion of the yoghurt before it is quite "done" and let the rest continue as normal. The refrigerated portion will continue to finish in the refrigerator while you consume the portion that was allowed to achieve that "perfect balance". You can estimate how much yoghurt you should set aside by noting how much you have have left over when it starts to become too sour for your taste. The next time you make yoghurt, set that amount aside (in the refrigerator) to be consumed after the first batch is gone.
Freeze what you cannot consume quickly
This option is far from ideal. You can slow or just about stop the fermentation process by freezing a portion of the yoghurt just after you make it. Unfortunately, when yoghurt is frozen then thawed, it can separate and become somewhat watery and grainy. Sometimes you can mix it back in, but it may not have that wonderfully creamy consistency of the original. You may have to try it and see. Defrosted yoghurt will still be healthy to consume, but some believe you will lose much of the health benefits as the live bacterial cultures can be destroyed by the freezing process.
Or keep it as a frozen treat
You may also consider not defrosting the frozen portion at all. Some people actually enjoy the experience of frozen yoghurt as a healthy snack alternative. If you consume some fresh and some frozen, you can think of it as two treats in one.