Welcome to the R7 club! Its an awesome camera but indoor light is always tough. You might want to consider the Godox TT685II-C Flash for Canon since it fits your budget perfectly. Just be careful with random off-brands, you really want E-TTL so the camera handles the power for you. Make sure to practice bouncing light off the ceiling before the wedding... it makes a huge difference!

I totally love my Godox TT350C Compact Flash! Its amazing for staying under budget. Definitely check the Strobist website to learn how E-TTL works before your sisters big day!

Building on the earlier suggestion, Godox is killer but I also totally love the Yongnuo YN600EX-RT II Wireless Flash for Canon for people on a strict budget! Its basically a clone of the high-end Canon flashes but for a fraction of the cost. The technical specs are amazing for the price... you get high-speed sync and a high guide number which you'll definitely need if you're shooting in a big wedding hall. Since you're worried about costs, definitely dont waste money on disposable batteries. Grab a pack of Panasonic Eneloop Pro AA Rechargeable Batteries because they recycle the flash so much faster. Ngl, waiting for a cheap flash to recharge while your sister is cutting the cake is pure stress. The R7 mount is plenty strong, just slide it in till it clicks and you're golden. Youre gonna have a blast with it!