@Dimotori for Form 3520, the threshold is US$100,000 aggregate from NRA person / Trust/Estate. Thus if the sum total of gift from your sister to you for the year is equal or greater than the threshold, then you need to recognize/report that gift. Ditto for your mother.
Transfer of funds from foreign accounts that you own or control do not come under 3520 regs.
I am assuming that both your sister and you mother are NRA individuals.
Note that this is only informational return and has no Tax Effect.
BTW -- if you have foreign accounts/assets that you own, control or have signature authority, these come under FBAR ( Form 114 , online only, at FinCen.gov ) and/or FATCA ( form 8938 along with your tax return )
Does this answer your query ? Is there more I can do for you ?
