Really everyone has their individual approach to things like this, it is totally up to you.
If you decide to stay later and suspect that the transfer from the car seat to bed will be disturbing then I would just brace yourself for that, expect it to happen and expect to need to spend some additional time helping her resettle. Expect that one night and possibly the next day too will be 'off track' but have faith that you will also return to your regular routine soon enough.
If you decide to leave early to get LO home in time for bed etc and to stay in routine then I would prepare your answer to family pressure/questions about this. Again brace yourself! If you believe it is the best thing for your LO and your family then be clear and confident in how you respond to external pressure, it's your decision, your LO, your night, your routine. You could for instance say "SIL, we love being at your place and always have such a wonderful time, it would be a real shame to have the night spoiled by dealing with an over tired, screaming baby all night long after such a lovely evening with you. I really do need to get her home, it's best for all of us so we are not exhausted for the rest of the holidays, I expect next year we will be able to stay later when she's a year older and not so effected by over tiredness. I'll really look forward to that."
Hope you have a lovely time whatever you decide!
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