ETA: I just read back and saw your budget... so that changes things somewhat...
(Note to self: 83 cm =32.7")
Good job keeping your child safely restrained all this time even though you don't own a vehicle.
Correct me if I am wrong, but it sounds like you have your 18mo FF? At 18mo she should really be rearfacing, and continue to RF for a long time yet. RF to the limits of the seat is recommended by Transport Canada and the minimum age we like to see kids RF is 2yo because of the very significant safety benefits of RF over FF for younger kids. For some more info, please ask us or see:
[ame="http://www.youtube.com/watch?v=kvyIv9QVRBE"]YouTube- Rear facing is safer[/ame]
http://www.joelsjourney.org/
I would get a seat which will RF as long as possible, then harness for a long time FF. The new models of the Scenera are actually a good option as they RF to 35lbs but they don't FF for very long.
ETA out of the ones I told you about I would really only leave *the First Years True Fit (Zellers - often on sale for about $150, online at BRU). I would also add to your list the newer models of the Cosco Alpha Omega 3in1s - they RF to 35 lbs, then FF harness to 50 lbs with 17.5" top slots which is a nice height, BUT these make a very POOR booster for the majority of kids so I would definitely NOT plan to use it as such. They can be found for a decent price at Zellers and Walmart. Zellers quite often has these on sale for $119-$139. Do make sure to check the stickers for a 35lb/50lb seat though, since there are still some 30/40 seats with low top slots around. Oh oh oh! And also look at the Evenflo Triumph Advance (NOT the Titan). It RF only to 30 lbs but you still have another 5 lbs to go, and has a nice tall shell, and then FF to 47lbs with pretty tall top slots. It's at Sears for $169.99, I think (some) Walmarts have them, and online at BRU for $159.99.
If you really can't go over $100, I would buy a new Scenera to RF in (~$90, Walmart), and then save up for a good FF seat for when that is outgrown.
Canadian Tire really has a pretty poor selection of car seat choices at this time.
ETA these may be too m uch over your budget though they are great seats so if you could extend the budget, they would be nice choices:
So, I would look at [well over $100: the various models of Sunshine Kids Radian (you would have to buy this online, most likely unless there is a small baby store near you which carries these)], TF moved above*, [the Graco MyRide (Walmart, Sears, online at BRU) - also over $200, sometimes on sale for ~$180ish], [over $200 as well: the Safety 1st Complete Air (Walmart or online at BRU)].
This thread will give you some stats:
http://car-seat.org/showthread.php?t=36056
You want to look at a higher weight limit, tall top slots and a tall shell. Most kids outgrow seats by height before weight, and your DD sounds fairly light, so you don't need to worry too much about getting a seat which will FF to 65 lbs vs say, a 50lb seat. We can give you more detailed reviews of the seats after you have a look around and see what you like.
If you are only going to FF, then we can suggest other seats which will last longer.
It might also help if we know the age, height and weight of your older DD, and what seat she currently rides in, as sometimes it makes more sense to pass a seat down and get a new one for the older child.
For your SIL: If you can fit the seats side by side, the center seat provides some additional protection, and I believe being installed next to another seat also provides some additional protection, so I would do that. It would also allow someone to ride in the back next to the babies. However, in many cases it is difficult to get 2 seats side by side, depending on the vehicle and the seats. So if she can't do it, installing both outboard is fine as well. My kids ride outboard in the smaller vehicles and side by side in the truck.