Going by contents of your post, it appears that, your Mother's SIL is in the property as a "Permissive Possessor", she cannot claim adverse possession.
If the property is your mother's self acquired, then SIL cannot claim Adverse possession.
If she claims ownership, she has to prove title, which point goes in your favour.
SIL is a trespasser if she continues in the premises.
Concept of adverse possession does not lie in her favour, further, if SIL claims ownership, she has to prove and establish the title flow in her favour by documentary proof (other than permissive possession given by your mother)