Given the issues with other family members...
I actually would put the house on the market asap.
Send a letter before action, giving them a time frame to shift it 'or else'. Make sure that LBA actually says 'this is a letter before action, you have x days to do xyz or else xyz will happen'.
Put the house on the market.
Let it be known to family members that your house IS on the market and they can verify this for themselves if they check listings. This is a very reasonable reason to want the car shifted, so relatives pissing and moaning about you wanting the car shifted because you're unreasonable/mean etc won't stand.
Investigate your options if the LBA is ignored - if your LBA states 21 days or 30 days then thats plenty of time to get legal advice and know what your next steps are.
Chances of your house selling in that time frame are pretty much zero.
If you DO go to court, 'I need it gone because in X months I won't live at the property' is a bloody good reason, even if 'I said it could sit there for a few months 10 years ago' seems not to be (it really really would be).
I'd also, in the meantime, between LBA and anything else, gather together any written evidence you have of asking for it to be shifted, promises it will be shifted/sold/etc, just in case you need that.