Welcome to the forum.
Boot is 44" wide at its widest point, and 24" long at its longest. However the 24" length only extends for a maximum of 36" so I don't think your pram will go in. There are some delicate parts in the boot and the microswitch on the LHS is vulnerable. If broken her roof won't work! Any longer than 36" and the useable length is down to about 21" (17" at its shortest point).
You may get the pram behind the rear seats, either across them or sticking up on one side. Careful not to damage the upholstery though!
Good luck, hope your visit to your sister goes well.
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