I love jigsaws as well. Like you, I get mine from charity shops and they get returned after completing them a couple of times. It is usually a winter occupation and it can get a bit addictive and a terrible time-eater!
It is always worth ensuring that it is made by a well known name - some of the cheaper ones are very badly made and the pieces bend and don't fit well.
I've always been lucky and not had any pieces missing. I think that is a lovely way to spend time together with your sister.
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