And now Crimea is part of Russia. So why you are talking about 1991? Ukraine has never been a member of the CSTO.
And it never will be member of NATO.
In May 2005 Georgia started consultations with Ukraine on their withdrawal from the Commonwealth of Independent States.
On 14 May 2008 Georgia's Foreign Ministry has announced its withdrawal from a 1995 CIS unified air defense agreement signed by a number of former Soviet republics, including Russia, on April 19, 1995. Georgia had previously withdrawn from the CIS Defense Ministers Council, although it formally remained in the CIS unified air defense system. On 19 August 2008 the CIS Executive Committee received a note from Georgia on its intention to pull out of the organization.
On 20 August 2008 Foreign Minister of Ukraine Vladimir Ogryzko said "The name of CIS envisages unity, friendship, but it should be analyzed whether this friendship is real or a curtain", said the minister and mentioned that Ukraine was not the member of CIS. "Ukraine is not the member of CIS Economic Court, has not ratified the Statute of CIS, that's why Ukraine can not be considered member of CIS in terms of international law. Ukraine is only a participating country, but not the member", he said. Nine full members were left in the CIS - Armenia, Azerbaijan, Belarus, Kazakhstan, Kyrgyzstan, Moldova, Russia, Tajikistan and Uzbekistan.
1995, Ukraine belongs to the Commonwealth of Independent States (CIS) air defense system
The Russia-Georgia War of 2008, recognition of South Ossetia and Abkhazia as independent states (demanded by Russia and denied by NATO), future NATO membership of Ukraine and Georgia, and NATO missile defense have all strained NATO-Russia relations and shown the weakness of the NATO-Russia Council.
http://ukraineun.org/en/press-cente...-and-sub-regional-organizations-csto-sco-cis/
Statement by the delegation of Ukraine at the UNSC debate on cooperation between the UN and regional and sub-regional organizations: CSTO, SCO, CIS
First of all, I refer to the Russian aggression in Ukraine and Georgia. The Russian Federation, one of the founding members of these organizations, continues to blatantly violate the UN Charter, the CIS and CSTO Charters, international law, multilateral and bilateral agreements.
And at this very moment these organizations are still pretending that there is no ongoing Russian aggression against Ukraine, no occupation of Crimea, no de facto occupation of Abkhazia and South Ossetia, no war crimes committed against Ukrainian and Georgian peoples.
Until then, unfortunately, we cannot agree that the CIS and CSTO are able to “make every effort to achieve pacific settlement of disputes” as enshrined in article 52 of the UN Charter.
In this regard I would like to commend the SCO members that during the conflict in Georgia in 2008 underscored the need to respect the territorial integrity of states. We wish the same approach be maintained vis-a-vis the temporary occupation of Crimea and the Russian aggression against Ukraine.