Someone i don’t want asked to be a bridesmaid!

I currently have 4 bridesmaid with the possibility of a 5th one. One of them has a sister and growing up I was close to both of them though we lost touch over the years. I met up with one of them recently after I got engaged. We had a great time and started taking about my wedding (she was already invited). I mentioned how my fiancé has more groomsmen than I have bridesmaids and she kinda offered to be one, in a joking but serious way. I thought it was a bit awkward but I was kinda thinking about asking her anyways so said of course. At the time her sister was abroad and was going to be for a few years so even though she was invited, she said she couldn’t make it. Anyways something came up so she’s coming home now and she literally texted me asking if she can be a bridesmaid (probably because her sister is). I haven’t seen her in 10 years and our personalities are so different now. I’m stressed because even though we were close growing up, her and her sister would constantly fight and literally abuse each other. It wasn’t great hanging out with them at the same time. I feel like asking like that is so rude. I don’t know if I even have room in my budget for another bridesmaid. I’m still waiting to hear back from the 5th girl who will already bring me over budget. She’s my best friend from college, who lives abroad and is having trouble getting a visa, so for her it’s worth it. What do I do???!