(1) Firstly, it should be made clear that the purpose of Kollel Iyun Hadaf is not to issue Halachic rulings. Therefore, what I will attempt to do now is merely to cite some of the sources relating to your question, in order to provide you with background information, but any practical questions should be addressed to a competent local Rabbi.

(2) Secondly, I am not sure what the source of your question is. The curse mentioned in our Gemara only applies to one who cohabitates with his mother in law, not to a person who is hosting her in his house. There is another Gemara that does relate to the latter, though. In Bava Basra 98b we are told that it is not proper for a son-in-law to live in his in-laws' house (because he might become too friendly with his mother-in-law).

(3) A similar Gemara appears in Kidushin 12b which states that one of the sins for which Rav administered punishment was a son-in-law who lives in his father-in-law's house. The Gemara then states that Neharda'ei disagree with the first opinion of the Gemara and maintain that Rav did not in fact punish for this. Tosfos DH b'Kulhu writes that this is what present-day sons-in-law rely on to live in the same house as the mother-in-law. Tosfos adds an additional reason to be lenient, because the reason couples live in the parents-in-law's house is because they gain from this - they save themselves having to pay rent, or benefit from other kinds of help from the in-laws. This proves that the son-in-law does not have his mind on his mother-in-law so there is no suspicion.

[In your case you would not be moving in with your mother-in-law for your own gain but rather on the contrary in order to help the rest of the family. However there is still an obvious reason why you would be moving in, so people would not be suspicious of immodest motives.]

(4) In addition, see Ra'avad to Rambam Hilchos Isurei Bi'ah 21:15, who writes that if the son-in-law has a separate place to sleep, this is permitted. So in your case, if there were separate sleeping quarters, this would be a reason to be lenient.

(5) Clearly the advanced age of your mother-in-law would provide additional logic to suggest that one is less apprehensive of any suspicion.

(6) It should also be pointed out that while the Rambam in Hilchos Isurei Bi'ah 21:15 cites this Gemara, nevertheless it is not cited in Shulchan Aruch. This might provide extra logic to argue that an arrangement could be reached to enable you do the Mitzvah of helping your mother-in-law and grandchildren.

Please let me know if I have not understood your question properly.

KOL TUV

Dovid Bloom