I would say it's a "normal" reaction, in that Frankie is probably upset by the crying and not verbal enough to express it, so he acts out physically. When Emma cries, Wyatt sometimes cries too (kind of fake cries, but it's because he's upset by her cries), and he always says "baby cry". I think it's normal for young toddlers to get distressed by babies crying, particularly a sibling that they see mom and dad taking care of. So I wouldn't worry that it's an abnormal reaction.
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As far as how to stop it, I would try to redirect his distress. Wyatt is very into "helping" with Emma (just like Frankie seems to be!), so when Emma cries, we ask Wyatt to go get a pacifier for her, or get her blanket for her, or something like that. Maybe giving Frankie a task that will "help" baby feel better will alleviate some of his distress. Or give him a different way to express his feelings...if he's hitting people because he's upset by the baby crying, try to explain to him a different way to deal with that emotion, like hitting a pillow, or ripping up paper, or yelling (if you are OK with that as an outlet for him).
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And, of course, if he's upset by the baby crying, give him things that calm him down when he's upset. For Wyatt, that is his pacifier and his blanket. For Franklin, maybe a bottle of milk? I think it's just a matter of teaching him how to "appropriately" express his feelings. So if he's feeling upset/frustrated/scared by the baby crying, tell him it's OK to feel that way when baby cries, but give him other ways to handle those feelings. Just my two cents, hope that helps!